Five Britons have been detained by the Iranian navy while sailing a yacht from Bahrain to Dubai, the Foreign Office (FCO) has said, BBC reported.
The sailors were crewing a boat belonging to Sail Bahrain when they were stopped on 25 November.
In a statement, the FCO said it was possible the crew, who are still in Iran, may have "strayed inadvertently into Iranian waters".
The five are understood to be safe and well and their families have been told.
It is not known where the sailors are being held nor which club they had come from, but the FCO did confirm they were on their way to take part in a Dubai-Muscat race. They are believed to have been sailing a yacht called the Kingdom of Bahrain.
FCO officials have had contact with Iran's ministry of foreign affairs and the Iranian embassy in the UK, while Foreign Secretary David Miliband said he hoped the issue would be resolved "soon".
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, she added.
Map of the Gulf
It is thought the Eid holiday could have delayed proceedings in Iran, said the BBC's Tehran correspondent Jon Leyne.
He added that with Iran feeling beleaguered over Western concerns about both its recent elections and its nuclear programme, the incident "could hardly [have come at] a worst moment".
Mr Miliband added: "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.
Stand-off
Sail Bahrain was recently launched by yachting company Team Pindar.
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.
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 released nearly two weeks later.
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.