Iran says it has lifted a detention order on a Swedish-owned, British-flagged oil tanker, but that it may not leave because of an ongoing inquiry.

Foreign ministry spokesman Abbas Mousavi tweeted that the Stena Impero was suspected of “violations and damages inflicted on the environment”.

The ship’s owner, Stena Bulk, said it was not aware of any formal charges.

Iran seized the Stena Impero in the Strait of Hormuz in July, accusing it of violating maritime rules.

The seizure came two weeks after an Iranian tanker was held off Gibraltar with the help of the Royal Marines.

That ship, now called the Adrian Darya 1, was suspected of violating EU sanctions on Syria but it was released by Gibraltar on 15 August.

The Stena Impero was passing through international waters in the Strait of Hormuz, a narrow waterway that connects the Gulf and the Indian Ocean, on 19 July when it was detained by Iran’s Islamic Revolution Guard Corps.

Video footage showed troops descending from a helicopter on to the deck.

A British Royal Navy frigate deployed in the Gulf tried to come to the tanker’s aid and warned the Iranians by radio that their actions were illegal, but that it was unable to reach the scene in time.

On Monday, the Iranian ambassador to the UK and the head of Iran’s Ports and Maritime Organisation said the vessel was free to go following the conclusion of judicial proceedings.

But Stena Bulk Erik Hanell said on Tuesday afternoon that despite the statements, the Stena Impero remained detained at anchor in the port of Bandar Abbas.

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