The crew were on a yacht backed by Team Pindar
Five Britons have been detained by the Iranian navy while sailing a yacht from Bahrain to Dubai for a race, the Foreign Office (FCO) has said.
Their Volvo 60 yacht backed by the UK's Team Pindar was stopped on 25 November.
The FCO said the crew - Luke Porter, Oliver Smith, David Bloomer, Oliver Young and Sam Usher - may have "strayed inadvertently into Iranian waters".
Dubai-Muscat Offshore Race organisers said the crew may have been "drifting" after experiencing propeller problems.
Dubai-Muscat Offshore Race organisers said the crew, who were participating in a race, 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 the Kingdom of Bahrain 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."
The five, who are still in Iran, are understood to be safe and well and their families have been told.
Mr Smith, 31, an engineer, from Southampton. His teammate Mr Bloomer is said to work as a sports broadcaster in Bahrain.
It is not known where the sailors are being held but the FCO did confirm they were on their way to take part in the Dubai-Muscat race.
The British Embassy in Tehran is demanding the immediate release of the five but has so far only had indirect contact with the crew members.
It is thought the Eid holiday could have delayed proceedings in Iran.
FCO officials have spoken with Iran's ministry of foreign affairs and the Iranian embassy in the UK, while Foreign Secretary David Miliband has asked for a phone conversation with his opposite number in Tehran.
The BBC's diplomatic correspondent Bridget Kendall said the FCO had wanted to keep the matter "private" in order to increase the chance of a resolution.
But after five days the details emerged and they had no option but to confirm the story.
Our correspondent said the timing was awkward, coming after the UK condemned Iran's plan to extend its nuclear programme.
The government feared Iran might see the detention as an opportunity for "extra leverage" in relation to the nuclear dispute, she added.
Mr Miliband said: "FCO officials immediately contacted the Iranian authorities in London and in Tehran on the evening of 25 November, 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," he said.
The 360-nautical mile Dubai-Muscat Offshore Sailing Race began on 26 November and ended two days later in the Omani capital's Bandar Al-Rawdah marina.
The Kingdom of Bahrain is owned by the Sail Bahrain project, which aims to promote the island as a yachting destination and was recently launched by Team Pindar.
In a statement, Team Pindar confirmed Kingdom of Bahrain was stopped by Iranian navy vessels, as it headed to the start of the race.
It added: "The boat may have strayed inadvertently into Iranian waters. The five crew members, all British nationals, are still in Iran."
Mr Smith took a degree in Ocean Science and Marine Navigation at the University of Plymouth.
He sailed on the university's 1st Team was later part of a team which came third in the Racing Division of the ARC (Atlantic Rally for Cruisers).
In March 2007 there was a prolonged stand-off between the UK and Iran after a 15-strong Royal Navy crew was detained by Iran's Revolutionary Guard.
The Iranians accused the crew of straying into its waters, but the British said they were in Iraqi territory.
They were pardoned and released nearly two weeks later by President Ahmadinejad.
In 2004, eight British servicemen were held in Iran after being seized in the Shatt al-Arab waterway, where they were training the Iraqi river patrol service.
In both instances, the crews were paraded on television by the Iranian authorities and Bridget Kendall said British diplomats are worried it might happen again in the latest case.