British Royal Marines captured the massive ship on suspicion that it was ferrying its cargo of 2.1 million barrels of crude oil to Syria, in contravention of European sanctions.

But speaking to state news agency IRNA, Abbas Mousavi, an Iranian government spokesman, said: “The legal pretexts for the capture are not valid...The release of the tanker is in all countries' interest.”

He threatened reciprocal measures against the UK if the tanker was not allowed on its way.

The warning came just a day after three Iranian boats were said to have impeded a British commercial tanker as it passed through the Strait of Hormuz on Wednesday – an act of apparent retaliation for the Gibraltar seizure.

The trio of boats, though to be Islamic Revolutionary Guard vessels, only backed down when HMS Montrose, a Royal Navy warship, arrived on the scene and trained it guns on them.

Jeremy Hunt, foreign secretary, said the escalating tensions now showed that money should be spent strengthening the Royal Navy.

The Tory leadership hopeful said more warships were needed if the country is to continue to defend its interests abroad and on the seas.