The crew were on their way to take part in the Dubai-Muscat yacht race
An Iranian official has said "serious" measures will be taken against a UK yacht crew if it is proved they had "evil intentions".
The five Britons are being detained by the Iranian navy after the Volvo 60 yacht was stopped on 25 November.
The Foreign Office said Luke Porter, Oliver Smith, David Bloomer, Oliver Young and Sam Usher may have "strayed inadvertently into Iranian waters".
The Team Pindar-backed yacht was sailing from Bahrain to Dubai.
Esfandiar Rahim-Mashaie, President Ahmadinejad's head of staff, told Iran's Fars news agency: "Judiciary will decide about the five... naturally our measures will be hard and serious if we find out they had evil intentions."
Foreign Secretary David Miliband said he was expecting the Iranians to make a statement later on Tuesday.
By Jon Leyne, BBC Tehran correspondent
Comments about possible hard and serious measures against them are particularly ominous because of the person they have come from, Esfandiar Rahim-Mashaie.
He is probably the closest ally of Mr Ahmadinejad in the government, very much his ideological mentor, so a real hardliner.
We will have to see how they play it, but there is always the danger of some sort of tussle within the machine, with some wanting a hard line, others thinking it might harm Iran's interests.
There are a couple of other issues. The foreign secretary is using diplomatic channels, he's hoping to speak to the Iranian foreign minster - but the Iranian foreign minister may not run far with the Revolutionary Guards because they basically think they are in charge.
Iran also has the most public holidays of any country in the world and they have two more coming up, so even in the normal run of things it is going to be a lengthy process.
He also said he hoped the matter would be resolved "soon" and that there was "no confrontation or argument".
Mr Miliband said: "This is a human story of five young yachtsmen. It's got nothing to do with politics, it's got nothing to do with nuclear enrichment programmes... it has no relationship to any of the other, bigger issues."
He added: "They were going about their sport and it seems they may have strayed inadvertently into Iranian waters."
The Foreign and Commonwealth Office (FCO) says it believes the five sailors are being held on the island of Sirri. It said it was seeking confirmation of this from the Iranian authorities. The sailors are understood to be safe and well.
Earlier the Iranian Ambassador met at the Foreign Office with the FCO's top civil servant and permanent under secretary, Peter Ricketts, to discuss the situation.
Dubai-Muscat Offshore Race organisers said the crew of the Kingdom of Bahrain yacht, who were preparing for 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 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."
David Young, said his 21-year-old son, Oliver, was a "fanatical yachtsman" with a great deal of experience and that he was certain his son would be able to "cope with this very well".
But Charles Porter, of Weston-super-Mare, who is the father of 21-year-old Luke, said he and his wife Beverley were concerned.
He said: "We are holding things together as a family at the moment. I haven't spoken to him since yesterday. He was as good as can be expected."
The fiancee of Sam Usher, 26, Nicola Drayton, said: "It's difficult but you just get on, you have no choice."
Luke Porter's parents say they have 'grave concerns' for his welfare
Of the remaining two captives, Mr Smith, 31, is an engineer from Southampton and his teammate Mr Bloomer is said to work as a sports broadcaster in Bahrain.
The FCO said the men had been 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 Mr 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.
Oliver Smith, 31, an engineer from Southampton, yacht's skipper
Oliver Young, 21, professional yachtsman from Plymouth
Luke Porter, 21, professional sailor from Weston-Super-Mare
David Bloomer, radio presenter based in Bahrain
Sam Usher, 26, owns sailing academy in Scarborough
The government feared Iran might see the detention as an opportunity for "extra leverage" in relation to the nuclear dispute, she added.
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 yacht is owned by the Sail Bahrain project, which aims to promote the island as a yachting destination and was recently launched by Team Pindar.
Team Pindar is owned by G A Pindar & Son Ltd, a family-owned print and publishing business based in Scarborough, North Yorkshire.
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 is not the first time British sailors have been detained after being accused of straying into Iranian waters.
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.
And 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.
IRAN'S MARITIME CLAIMS
In 1993 Iran informed the UN of its claims over a territorial sea, a contiguous zone and an exclusive economic zone (EEZ) extending to the continental shelf.
Territorial sea: Iran claims sovereignty over a belt of sea, measured 12 nautical miles from its baseline. This extends to the air space above it as well as to its bed and subsoil
Contiguous zone: An area adjacent to the territorial sea with an outer limit of 24 nautical miles from the baseline. The Iranian government claims the right to take steps to prevent the infringement of laws and regulations
EEZ and the continental shelf boundary: Within the EEZ, which extends out to the continental shelf line, Iran claims exclusive rights to all natural resources, research and the building of structures.
Source: Bureau of Oceans and International Environmental and Scientific Affairs, US Department of State
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