Five British sailors were being detained in Iran today after their racing yacht was stopped by the Iranian navy, the Foreign Office said.
Their boat, the Kingdom Of Bahrain, owned by Sail Bahrain, was stopped by Iranian naval vessels last Wednesday as it sailed from Bahrain to Dubai.
The crew members were named last night as Bahrain-based radio presenter David Bloomer, Oliver Smith, 31, from Southampton, Oliver Young, 21, from Plymouth, Sam Usher and Luke Porter, 21.
The sailors are understood to be safe and well. Their families have been informed.
The Foreign Office said the yacht may have "inadvertently" strayed into Iranian waters.
The sailors were heading to Dubai to join the Dubai-Muscat Offshore Sailing Race.
Their detention will further raise tensions between Iran and the West.
The country has come under increasing pressure in response to its plans to build 10 new nuclear fuel plants.
Foreign Secretary David Miliband said officials had contacted their Iranian counterparts as soon as they became aware of the situation.
He explained: "FCO (Foreign and Commonwealth Office) officials immediately contacted the Iranian authorities in London and in Tehran on the evening of November 25, both to seek clarification and to try and resolve the matter swiftly.
"Our ambassador in Tehran has raised the issue with the Iranian foreign ministry and we have discussed the matter with the Iranian embassy in London. I hope this issue will soon be resolved.
"We will remain in close touch with the Iranian authorities, as well as the families."
Sail Bahrain was recently launched by yachting company Team Pindar.
The boat had been due to take part in the 360-mile Dubai-Muscat Offshore Sailing Race which was due to start last Thursday.
A statement later issued by Team Pindar read: "On November 25, Sail Bahrain's Kingdom of Bahrain Volvo 60 racing yacht was stopped by Iranian navy vessels, as it was making its way from Bahrain to the start of the Dubai-Muscat Offshore Sailing Race.
"The boat may have strayed inadvertently into Iranian waters.
"The five crew members, all British nationals, are still in Iran.
"All are understood to be safe and well and their families have been informed."
Shadow foreign minister David Lidington called for the sailors to be released immediately.
He told the BBC: "I think it is completely unjustifiable and I believe Iran should release these people from captivity without delay.
"There was no justification for them being arrested in the first place."
Dubai-Muscat Offshore Race organisers said the sailors may have been "drifting" after experiencing propeller problems.
Louay Habib, from the Dubai Offshore Sailing Club, told the BBC the shore crew for the boat had said "there was no wind at the time, and they told us that they were organising for a tow to come and get them".
He added: "It's purely speculation but they would have probably been drifting... in 10 hours they could well have strayed into Iranian waters."
Mr Smith, is an experienced sailor. He completed a degree in Ocean Science and Marine Navigation at the University of Plymouth whilst sailing on the University Mens' 1st Team.
He then spent six months skippering a 50ft survey yacht and doing delivery work in South America and the Caribbean.
He was part of a team which came third in the Racing Division of the ARC (Atlantic Rally for Cruisers), and in 2003 went on to work for Pure Sailing as the boat captain and skipper for a Volvo 60 raceboat.
Mr Bloomer, who is believed to hold dual British and Irish nationality and is said to be in his 60s, was due to broadcast updates on the boat's progress in the race.
Mr Porter's father Charles said he had spoken to his son yesterday.
He told The Sun newspaper: "He was in good spirits and he said he was being well looked after."
In a brief conversation, he said his son had told him that the crew had accidentally strayed 500 yards into Iranian waters near the island of Sirri.
Mr Young's father David told the thisisplymouth website: "We're confident this will be resolved very quickly. We're all concerned and the sooner they are released the better. It is dragging along longer than we thought it would do.
"It's just a worry that there are diplomatic stresses at the moment. They are under international pressure. We just hope they're not used as a bargaining chip."
On November 19, Mr Usher posted a message on Facebook which read: "Hi Lads, see you in a few days for the event. hope all has been going ok!"
Ben Wallace, chairman of the British-Iranian All-Party Parliamentary Group, said the actions of the Iranian authorities may be a deliberate move to gain political leverage.
The Tory MP for Lancaster and Wyre told the BBC: "A normal nation would stop a yacht, check it out, maybe take it back to shore and get rid of them. But this is five, six days into the incident. That is much more worrying, that is more deliberate."
He continued: "This is more than just saying: 'Get out of our backyard' this is odd behaviour, but is in line with Iranian behaviour. When (Iran) wants to make a point it takes hostages."
Children, Schools and Families Secretary Ed Balls, speaking on GMTV, said the Cabinet would receive an update on the situation when it met later.
"Everything is being done to make sure that these guys are safe and they can come back soon," he said.
"We do not know exactly what happened, whether they did go into Iranian waters. We need to get the facts but also to get them back."
The incident is not the first time that Britons have been seized by the Iranian authorities while at sea.
Iran took 15 British service personnel hostage on March 23 2007 and held them for 13 days.
The Royal Navy sailors and Royal Marines, from the frigate HMS Cornwall, were taken at gunpoint by Iranian warships during a routine search of a cargo ship in the northern Persian Gulf.
Iran alleged the frigate had strayed into Iranian territorial waters but the Ministry of Defence denied the allegation and said the ship was in Iraqi waters.
Footage of crew members was broadcast by Iranian television during their kidnap ordeal.
Iranian president Mahmoud Ahmadinejad agreed to pardon and free them on April 4 2007.