The Iranian supertanker crew members who were arrested off the coast of Gibraltar have been conditionally bailed without charge.
The ship was seized last week in an operation involving British Royal Marines, after it was suspected of taking crude oil to Syrian president Assad’s regime in breach of EU sanctions.
Royal Gibraltar Police arrested the captain and his deputy on Thursday, while two second officers of the Grace 1 tanker were held on Friday.
The arrest of the four men, who are Indian nationals, came after a Royal Navy warship drove away Iranian patrol boats attempting to impede the progress of a British tanker through the Strait of Hormuz.
Police have said the investigation related to the arrests was ongoing and the vessel remained in detention.
Iran has demanded the immediate release of its oil tanker captured off the coast of Gibraltar, warning the UK: “This is a dangerous game and has consequences.”
A senior Iranian cleric, Kazem Sedighi, has also warned Britain will soon be “slapped in the face” for “daring” to capture the Iranian supertanker Grace 1.
Tehran wants tough sanctions lifted which were imposed by the US as Washington tries to renegotiate the Iranian nuclear agreement which it quit in 2018.
The 2015 accord was designed to curb Iran’s nuclear ambitions in return for relief from sanctions.
But since the US left the deal, it has imposed new penalties, hitting Iran’s economy and raising Washington-Tehran tensions.
Prime Minister Theresa May’s official spokesman has said discussions were ongoing between UK and US about increasing their military presence in the Gulf.
HMS Duncan, a Type 45 Destroyer, will sail to the Gulf in the coming days after recently completing NATO exercises in the Black Sea.
The vessel will operate alongside the Royal Navy frigate HMS Montrose, and US allies in the Gulf, but she will not be part of an international maritime coalition proposed by Washington.
It is understood that HMS Duncan was always due to travel to the region to give some relief to HMS Montrose.
However, her deployment has been brought forward as a result of the deepening crisis with Iran.
Ministers in London have urged Tehran to “de-escalate” the situation.
Dr Simon Mabon, international relations lecturer at Lancaster University, told Sky News: “Iran is engaged in a very high stakes poker game with the United States, and increasingly now with the United Kingdom.
“But the UK is also caught in a really delicate position. We know that Iran has been reaching out to European powers to save the nuclear deal.
“The UK hasn’t really stood up to the United States strongly enough for Iran, but it’s not been able to offer any economic benefits with Europe for the Iranians either.
“The UK is caught in a difficult position and it’s quite easy to see the UK coming quite badly out of this.”