The Strait of Hormuz, shipping, and law

Freedom of passage through the Strait is a key issue for all maritime nations, writes Professor Marc Weller, Director of the International Law Programme at Chatham House.

Explainer

Published 13 April 2026

Updated 16 April 2026 — 7 minute READ

Image — A vessel heading towards the Strait of Hormuz. Photo by Shady Alassar/Anadolu via Getty Images

Professor Marc Weller

Director, Global Governance and Security Centre, International Law Programme

On Sunday, President Donald Trump announced a blockade against shipping ‘trying to enter, or leave, the Strait of Hormuz.’

This move seems to aim at punishing Iran for having failed to agree to what Vice President JD Vance termed the ‘final and best offer’ for a peace settlement that he put forward during talks in Islamabad.

The temporary ceasefire proposed by Pakistan had provided for the lifting of Iranian restrictions on maritime movements through the Strait ‘as a goodwill gesture’.

This has not occurred, amid dispute about the application of the cease fire to Israel and its war in Lebanon.

Act of war

A blockade is an act of war. Its imposition compounds the fact that the US and Israel have launched an unlawful war against Iran. It also threatens the already fragile truce. 

Moreover, President Trump’s initial announcement seemed to suggest that it would cover all shipping through the Strait.

This would have made the Gulf states, and those depending on their oil and gas, its principal victims, rather than Iran.

US Central Command has now clarified that it will ‘not impede freedom of navigation of vessels transiting the Strait of Hormuz to and from non-Iranian ports.’

This clarifies that a traditional blockade is intended, trying to strangulate only the economy of the opponent and forcing a surrender, rather than stopping all traffic through the Strait altogether, which would clearly be unlawful.

President Trump’s initial announcement was also directed against the new Iranian practice to sell passage through the strait for a fee of up to $2 million. ‘No one who pays an illegal toll will have safe passage on the high seas,’ he added.

This would expose third-party tankers to arrest and seizure by US forces beyond the Strait.

But would the US really capture an Indian or Chinese super-tanker if they had paid the Iranian toll, or entered its ports or coastal areas? This would be a very significant escalation of the conflict, and Washington may well hesitate in making good its threat. 

A map showing the Strait of Hormuz
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Made with Natural Earth data.

The right of passage through the Strait

Freedom of passage through the Strait of Hormuz is a key issue for all maritime nations. The Strait controls shipping in the order of around 100-140 major vessels passing before the war per day.

When the 1982 United Nations Convention on the Law of the Sea (UNCLOS) was negotiated, a critical deal was struck reflecting this fact. 

Freedom of passage through the Strait of Hormuz is a key issue for all maritime nations.

The convention accepted that coastal states can lawfully extend their territorial sea from the previously accepted limit of three nautical miles (nm) to 12 nm. This placed some 138 additional straits that are less than 24 nm wide under the jurisdiction of one or more coastal states.

The Strait of Hormuz, with a width of 21 nm at its narrowest point, is covered by the territorial seas of Iran and Oman respectively.

In exchange, the coastal states had to accept that a special legal regime would apply to straits used for international navigation. While the coastal states enjoy sovereignty over their territorial seas in most aspects, an original limitation to that sovereignty applies – they must accept an enhanced right of ‘transit passage’ for shipping of all nations.

This right goes further than the traditional right of ‘innocent passage’ granted to shipping through the territorial sea of any state. Innocent passage allows for some interference with passing shipping in accordance with local law, for instance for the protection of the marine environment or regulation of fisheries.

The US correctly argues that transit passage has become accepted as a firm right of all states in international custom, also binding on non-parties.

Crucially, the coastal state may suspend the right of passage if it judges that demands of its national security so require.

In contrast, given the lack of other viable routes, transit passage guarantees un-suspendable passage to all ships that may not be ‘impeded’ in any way by the coastal state. That right applies in peace and war, although with some necessary qualifications where the direct participants in an armed conflict are concerned.

The positions of the parties

Neither the US nor Iran is a party to UNCLOS. The US correctly argues that transit passage has become accepted as a firm right of all states in international custom, also binding on non-parties. Iran asserts that it need only grant the more limited, traditional, right of innocent passage,  which can be suspended. It also claims that foreign warships must coordinate access with its authorities.

Oman has ratified UNCLOS, but has added statements affirming its ‘full sovereignty over its territorial sea’, and seeks to reserve its right to require prior permission for passage of warships.

However, UNCLOS rules out reservations of this kind. The US Navy has conducted a ‘freedom of navigation programme’ since 1979, enforcing the right of unimpeded passage.

This has regularly included unannounced passage of warships through the Strait of Hormuz. 

During the present truce Washington claims to have sent two guided missile destroyers through the Strait, to emphasize this point and to prepare for an operation to clear the strait of mines.

Overall, the bargain of allowing all coastal states to extend their territorial seas was conditioned on universal acceptance of the regime of transit passage. Moreover, even if there could be doubt in relation to the passage of warships, which is not really the case, this would not affect the traffic of oil and gas tankers at issue in this instance.

Impact of the armed conflict

Kazem Gharibabadi, the Iranian Deputy Foreign Minister for Legal and International Affairs, claimed earlier in the conflict that ‘we are now in a state of war, and wartime conditions cannot be governed by peacetime rules.’

The US-Israeli attack on Iran clearly brought an international armed conflict into being. This turns the Strait of Hormuz into a ‘belligerent strait.’

The US-Israeli attack on Iran clearly brought an international armed conflict into being. This turns the Strait of Hormuz into a ‘belligerent strait.’

While the conflict lasts, Iran would be entitled to attack US or Israeli warships under the law of maritime warfare. This might include convoys of merchant ships conducted by US warships.

Direct attacks on merchant vessels of the two belligerents and on neutrals are, however, prohibited. US and Israeli-flag merchant vessels cannot simply be sunk, although Iran could seize them, along with neutral shipping carrying contraband.

Iran initially effectively blocked passage through the Strait for all maritime commerce altogether. However, this action was clearly and unambiguously rejected by the UN Security Council (UNSC) as a ‘serious threat to international peace and security.’

At a meeting of the Council of the International Maritime Organization (IMO) in London, Iran later claimed to have adopted only ‘necessary and proportionate measures to prevent aggressors and their supporters from exploiting the Strait of Hormuz to advance hostile operations against Iran.’

Of course, the concept of ‘their supporters’ is not a legal one, and would leave it up to Iran to determine arbitrarily which state can be excluded from passage.

Position of the Gulf states and their allies

It is difficult to deny that the Gulf states, and those actively defending them from attack, are involved in an armed conflict. However, the UNSC, in its resolution 2817 (2026), has confirmed that the littoral Gulf states ‘are not parties to the hostilities.’

The council thus appears to confirm that only Iran, the US and Israel are to be treated as belligerents in a fuller sense.

Such temporary measures cannot infringe on the iron-clad guarantee to shipping of all nations of unimpeded passage through straits used for international navigation under all circumstances.

This means that the shipping of Gulf states and their allies, including the UK, should have remained unaffected by any restrictions the war might impose on the right of passage on the belligerents.

This view reflects the sensible proposition that Iran should not be entitled to deprive the Gulf states of their navigational rights simply by subjecting them to what the UNSC has determined in Resolution 2817 are ‘egregious attacks.’

Convoys and mine clearance

How would all this affect an international operation to clear mines and escort shipping? If the conflict resumes, the US would expose its own naval forces, and any vessels it might escort, to attack by Iran according to the law of naval warfare in the Strait.

Third states, such as the UK, might point to the fact that they are not to be classed as belligerents and can therefore participate in convoying neutral shipping.

UK forces have carefully limited their involvement to collective missile and drone defence focused on the territory of the Gulf states under attack. The use of UK airbases by US forces has been restricted to action only against the sources of such assaults in Iran.

As mentioned, the UNSC has confirmed that the littoral Gulf states are ‘not parties to the hostilities.’ Hence, neither are the states acting at their request in support of their right of collective self-defence.

Accordingly, the UK can rely on a clause in UNSC Resolution 2817 confirming the right of states to ‘defend their vessels from attacks and provocations.’

There would be nothing to prevent like-minded third states from clubbing together in a collective operation in favour of their respective flag-vessels. However, the UNSC failed to grant more specific Chapter VII authority for such a mission - authorizing the use of force to ensure passage should it be opposed by Iran - due to a Russian/Chinese veto.

In truth, there seems to be a consensus that any collective action of that kind should await a more definite ceasefire. Iran might consider early action of this kind as an intervention in the balance of forces and might oppose it forcibly. 

The proposed new Iranian scheme of passage

Iran’s scheme of permitting passage of ‘non-hostile vessels’ under  a regime of ‘coordination with Iranian authorities’ has raised international opposition and incurred the wrath of President Trump in the form of his newly announced blockade. 

It involves moving the sea lane from the centre of the strait towards its own coastline, on the landward side of the island of Larak. There, ships are required to stop to undergo inspection by Revolutionary Guards forces.

Iran might argue it is entitled to inspect ships for contraband. However, this claim is abusive. Virtually all traffic, around 95 per cent, through the Strait is tankers that can only carry oil and liquid gas and bulk carriers for fertilizers and similar cargo, rather than military materiel.

Even if it could be argued that such commodities might make an ‘effective contribution’ to the war effort, as the definition of contraband would require, these shipments are not directed towards the US as a party to the conflict.

Iran might also rely on the provisions of the law of the sea, which exceptionally permits inspections on the high seas to verify the nationality of a vessel.

The UK and other states have invoked this provision over the Russian shadow fleet, where there may be well-founded suspicion that a particular vessel is changing its flag-state nationality to evade lawful sanctions.

It appears that Iran charges no tariff to what it regards as friendly states. This offends against the express legal requirement that the right of passage must not be subjected to discriminatory considerations.

However, an argument of this rationale can hardly justify stopping over a hundred vessels a day in the Strait of Hormuz, all engaged in lawful trade, each with easily verifiable pedigrees and run by established international maritime companies.

Evidently, charging a ‘toll’ is in further violation of the transit passage regime. In fact, even in relation to the lesser regime of innocent passage which Iran (wrongfully) seeks to apply, the law of the sea quite expressly prohibits the charging of any fee simply for right of passage.

Finally, it appears that Iran charges no tariff to what it regards as friendly states. This offends against the express legal requirement that the right of passage must not be subjected to discriminatory considerations.

Defending the global interest

Iran cannot unilaterally re-route traffic away from the established sea lanes deeper into its territorial waters. Iran is a member of the IMO, and sea lanes through major straits are established in consultation between the relevant coastal states and the IMO.

At one point, in stark contrast to his present view, President Trump hinted that a toll scheme might be acceptable, provided the US shares in the income generated – a proposition that was reportedly raised by Trump’s emissaries in the talks in Islamabad.

The key point remains that neither the US nor Iran can impose the extent and modalities of a right of passage through the Strait of Hormuz. This is a matter of global regulation affecting the rights of all states. These rights cannot be diminished or negotiated away by two states.

This does not exclude short-term solutions on the way to a permanent cease-fire. This could feature face-saving elements for Iran, such as joint spot checks performed together by Iranian and international personnel without interfering with the uninterrupted continuation of passage.

However, such temporary measures cannot infringe on the iron-clad guarantee to shipping of all nations of unimpeded passage through straits used for international navigation under all circumstances. Otherwise, maritime freedom would be in severe jeopardy at chokepoints all around the globe.