The captain of a tanker seized while carrying Iranian oil has said Royal Marines used "brute force" in detaining his ship.
Earlier this month, UK forces helped authorities in Gibraltar who believed the tanker was carrying oil to a Syrian refinery in breach of EU sanctions.
The captain said marines made his unarmed crew kneel on the deck at gunpoint.
The MoD said the seizure complied with "international rules and norms".
On 4 July, about 30 marines from 42 Commando were flown from the UK to Gibraltar to help detain the tanker and its cargo, at the request of the Gibraltar government.
The vessel's captain, an Indian national who asked not to be named, said he was radioed a police request to board his ship and lowered his ladder.
But before anyone could board, a military helicopter landed on the ship in a "very dangerous" move, he said.
He told the BBC he identified himself as the captain but the marines ignored him and instead pointed their guns and shouted "look forward, look forward".
He said: "They didn't care whether I was master… there was no regulations… we had 28 unarmed crew. I was in a state of shock, everybody was in a state of shock.
"How do you come on a ship like this with armed forces and such brute force. For what reason?"
He said the marines could have boarded the ship and simply told him he had been arrested.
The captain agreed to speak to the BBC on condition his name was not used, saying he and his crew had been advised by Indian High Commission officials to remain anonymous while legal proceedings were ongoing.
Asked whether he felt there was anything illegitimate about his ship or the cargo, he said he had "followed company procedures", adding he did not know about the EU sanctions against Syria.
The captain has since been arrested and bailed by authorities in Gibraltar.
The MoD said the operation on Grace 1 was a "standard boarding" and that British armed forces were "held to the highest standards of professionalism".
Royal Gibraltar Police said the marines had acted in support of its officers and applied "the minimum use of force" to ensure officers could get on board, deal with the crew and take control of the vessel.
It added that the ship's four senior officers had been interviewed under caution.
The Gibraltar government said it had evidence which contradicted statements made by the captain and would release its own statement soon.
It said it had reason to believe the ship was carrying Iranian crude oil to the Baniyas Refinery in Syria, which the EU says provides financial support to the Syrian government. The refinery has been subject to EU sanctions since 2014.
The seizure of the tanker sparked a diplomatic crisis between the UK and Iran, which has escalated over the past four weeks.
On 19 July, British-flagged oil tanker Stena Impero was seized by Iran's Revolutionary Guard in the key shipping route, the Strait of Hormuz.
Tehran said the vessel was "violating international maritime rules" but the UK called the incident an example of "state piracy".
Both British Foreign Secretary Dominic Raab and Iran's ambassador to the UK have ruled out an exchange of the two seized ships.
Mr Raab said: "This is not about some kind of barter. This is about international law and the rules of the international legal system being upheld."
Iranian diplomat Hamid Baedinejad tweeted it was "impossible to advance a quid pro quo or barter exchange".
Impossible to advance a quid pro quo or barter exchange of detained UK and Iranian ships as some British media suggest. UK has illegally detained the ship carrying Iranian oil while the British ship is detained for violating some key safety/security regulations in Hormoz Strait.— Hamid Baeidinejad (@baeidinejad) July 29, 2019
UK-Iran tension in the Gulf
- 4 July: Royal Marines help authorities in Gibraltar seize a tanker carrying Iranian oil, because of evidence it was heading to Syria in breach of EU sanctions
- 9 July: The UK raises the threat to British shipping in Iranian waters in the Gulf to "critical"
- 10 July: A Royal Navy ship warns off Iranian boats attempting to impede a British oil tanker in the region
- 19 July: British-flagged oil tanker Stena Impero is seized by Iran's Revolutionary Guard
- 28 July: The Royal Navy sends a second warship to the Gulf to protect British ships