The captain of an Iranian oil tanker that was seized by royal marines earlier this month has claimed the British soldiers used “brute force” when they detained the ship.

The Grace 1 tanker was detained by authorities in Gibraltar with the help of marines on the suspicion that it was ferrying a cargo of more than 2 million barrels of crude oil to Syria, in contravention of European sanctions.

The captain of the Grace 1, an Indian national who asked to remain anonymous, told the BBC that the royal marines forced his crew to kneel at gunpoint after landing on the ship in a helicopter.

“There was no regulations… we had 28 unarmed crew. I was in a state of shock, everybody was in a state of shock,” he said.

He added: “How do you come on a ship like this with armed forces and such brute force. For what reason?”

The captain described the helicopter landing as “a very dangerous move”, and said he had “followed company procedures”.

In photos: British-flagged tanker seized by Iran Show all 13 1 /13 In photos: British-flagged tanker seized by Iran In photos: British-flagged tanker seized by Iran A speedboat of Iran's Revolutionary Guard moving around the British-flagged oil tanker Stena Impero Tasnim News Agency/AP In photos: British-flagged tanker seized by Iran Crew members of the British-flagged tanker Stena Impero, that was seized by Iran in the Strait of Hormuz EPA/IRIB News Agency In photos: British-flagged tanker seized by Iran A satellite image of the Iranian port city of Bandar Abbas reveals the presence of the seized British oil tanker, the Stena Impero on July 22 Maxar Technologies/Reuters In photos: British-flagged tanker seized by Iran Iranian Revolutionary Guard Corps (IRGC) claims to have seized Stena Impero at the Strait of Hormuz with 23 crew on board on 20 July EPA/IRIB News Agency In photos: British-flagged tanker seized by Iran Iranian Revolutionary Guards patrolling around the captured British-flagged tanker Stena Impero as it's anchored off the Iranian port city of Bandar Abbas AFP/Getty In photos: British-flagged tanker seized by Iran epa07733612 A handout photo made available by Iranian state-run IRIB News Agency on 22 July 2019 shows crew members of the British-flagged tanker Stena Impero, that was seized by Iran in the Strait of Hormuz on 19 July 2019. Iranian Revolutionary Guard Corps (IRGC) claims to have seized Stena Impero at the Strait of Hormuz with 23 crew on board on 20 July 2019. EPA/IRIB NEWS AGENCY / HANDOUT HANDOUT EDITORIAL USE ONLY/NO SALES IRIB NEWS AGENCY / HANDOUT EPA/IRIB News Agency In photos: British-flagged tanker seized by Iran A satellite image of the Iranian port city of Bandar Abbas reveals the presence of the seized British oil tanker, the Stena Impero on July 22 Maxar Technologies/Reuters In photos: British-flagged tanker seized by Iran A speedboat of the Iran's Revolutionary Guard moves around a British-flagged oil tanker Stena Impero AP In photos: British-flagged tanker seized by Iran epa07733614 A handout photo made available by Iranian state-run IRIB News Agency on 22 July 2019 shows crew members of the British-flagged tanker Stena Impero, that was seized by Iran in the Strait of Hormuz on 19 July 2019. Iranian Revolutionary Guard Corps (IRGC) claims to have seized Stena Impero at the Strait of Hormuz with 23 crew on board on 20 July 2019. EPA/IRIB NEWS AGENCY / HANDOUT HANDOUT EDITORIAL USE ONLY/NO SALES IRIB NEWS AGENCY / HANDOUT EPA/IRIB News Agency In photos: British-flagged tanker seized by Iran In this photo released by state-run IRIB News Agency, which aired on Monday, July 22, 2019, shows cooks in the galley of the British-flagged tanker Stena Impero that was seized by Tehran in the Strait of Hormuz on Friday. The Associated Press cannot independently verify the condition of the crew members, but in the video they looked to be in good health and it didn't appear as though they were being filmed under duress. (IRIB News Agency via AP) AP In photos: British-flagged tanker seized by Iran epa07733619 A handout photo made available by Iranian state-run IRIB News Agency on 22 July 2019 shows crew members of the British-flagged tanker Stena Impero, that was seized by Iran in the Strait of Hormuz on 19 July 2019. Iranian Revolutionary Guard Corps (IRGC) claims to have seized Stena Impero at the Strait of Hormuz with 23 crew on board on 20 July 2019. EPA/IRIB NEWS AGENCY / HANDOUT HANDOUT EDITORIAL USE ONLY/NO SALES IRIB NEWS AGENCY / HANDOUT EPA/IRIB News Agency In photos: British-flagged tanker seized by Iran epa07733613 A handout photo made available by Iranian state-run IRIB News Agency on 22 July 2019 shows crew members of the British-flagged tanker Stena Impero, that was seized by Iran in the Strait of Hormuz on 19 July 2019. Iranian Revolutionary Guard Corps (IRGC) claims to have seized Stena Impero at the Strait of Hormuz with 23 crew on board on 20 July 2019. EPA/IRIB NEWS AGENCY / HANDOUT HANDOUT EDITORIAL USE ONLY/NO SALES IRIB NEWS AGENCY / HANDOUT EPA/IRIB News Agency In photos: British-flagged tanker seized by Iran epa07733616 A handout photo made available by Iranian state-run IRIB News Agency on 22 July 2019 shows crew members of the British-flagged tanker Stena Impero, that was seized by Iran in the Strait of Hormuz on 19 July 2019. Iranian Revolutionary Guard Corps (IRGC) claims to have seized Stena Impero at the Strait of Hormuz with 23 crew on board on 20 July 2019. EPA/IRIB NEWS AGENCY / HANDOUT HANDOUT EDITORIAL USE ONLY/NO SALES IRIB NEWS AGENCY / HANDOUT EPA/IRIB News Agency

The captain’s account of the seizure contradicts a statement from police in Gibraltar, who claimed that “minimum force” had been used.

An Ministry of Defence spokesperson said the seizure “was a standard boarding that was carried out in full compliance with international rules and norms.”

They added: “Our armed forces are held to the highest standards of professionalism.”

Iran has denied the Grace 1 was heading for Syria, and responded to the seizure by detaining a British-flagged tanker in the Strait of Hormuz, a vital oil shipping route that has become a flashpoint in a deepening standoff between Tehran and the west.

Tensions have been high in the strait in recent months amid a deepening row between Iran and the US, which was sparked by President Donald Trump’s decision to abandon a landmark nuclear deal and reintroduce sanctions.

Britain and the US have accused Iran of carrying out a series of attacks on tankers in the strait – a route that connects Middle Eastern oil suppliers to the rest of the world.

Two weeks after the Grace 1 was detained, the British-flagged Stena Impero was making its way through the Strait of Hormuz when it was suddenly surrounded in the sea and in the air by Iranian Revolutionary Guard troops. Speedboats circled as masked men, armed with guns, rappelled from helicopters to take control of the ship.

The Revolutionary Guard said Stena Impero had been seized for “not observing international marine rules”. But Iranian officials have since suggested it was detained in retaliation for the seizure of the Grace 1.

Britain is currently seeking to set up a European-led maritime protection mission to ensure safe shipping in the strait, and is now sending a royal navy escort with all British-flagged ships passing through it. HMS Montrose, a British frigate now in the area, carried out the first mission under the new policy last week.

France has given cautious backing to the idea of a European-led mission, according to Reuters, and the US formally asked Germany to join the mission on Tuesday.

“We’ve formally asked Germany to join France and the UK to help secure the Straits of Hormuz and combat Iranian aggression. Members of the German government have been clear that freedom of navigation should be protected... Our question is, protected by whom,” a US embassy spokesperson in Berlin told Reuters.

There is considerable opposition among Germany’s Social Democrats (SPD), junior partners in conservative chancellor Angela Merkel’s ruling coalition, to getting involved in a US-led mission.

“The German government has already rejected participation in the US military mission, Operation Sentinel, to protect shipping in the Strait of Hormuz,” said Nils Schmid, a foreign affairs spokesman for the SPD parliamentary party.

“It should stay like that. Otherwise, there is a risk of being pulled into a war against Iran on the side of the United States,” he added in an interview with Stuttgarter Zeitung.