The tanker's operator thanked the British authorities, saying help was requested after seven stowaways who had boarded in Lagos, Nigeria, turned hostile.

Troops from the Special Boat Service, Britain's elite naval special forces unit, boarded the Greek-operated Nave Andromeda near the Isle of Wight off southern England after the master of the ship said he was concerned about the safety of his crew.

"The seven men have been arrested on suspicion of seizing or exercising control of a ship by use of threats or force," police said.

The men are in police custody.

Defence Secretary Ben Wallace and Home Secretary Priti Patel authorised the armed forces to board the tanker "to safeguard life and secure a ship that was subject to suspected hijacking", the defence ministry said.

Sources with knowledge of the situation aboard the vessel said it was not a hijacking or hostage taking.

"The UK authorities had been advised by the Master that stowaways had been found on board and that he was concerned for the safety of the crew due to the increasingly hostile behaviour of the stowaways," Athens-based Navios Tanker Management said.

"Happily no crew members were injured and all are safe and well," it said.

"Navios Tanker Management wish to thank all the UK Authorities involved in this operation for their timely and professional response."

The seven were detained in the operation involving the SBS, the navy, coastguard and police.

"Navios would also like to pay tribute to the Master of the Nave Andromeda for his exemplary response and calmness and to all the crew for their fortitude in a difficult situation," the ship operator said.

The defence ministry declined to confirm or deny the involvement of the SBS - in line with British government policy of not commenting on special forces operations.

But a source with knowledge of the matter said the SBS were involved.

SBS operations are usually classified.

Australian Associated Press