The Royal Navy says satellite data proves 15 personnel being held in Iran were 1.7 nautical miles inside Iraqi waters when they were seized.
Vice Admiral Charles Style said the Iranians had given two different positions for the personnel - the first inside Iraqi waters in the Gulf.

He said they had been "ambushed" after the search of a vessel and that their detention was "unjustified and wrong".

Iran insists the group were in its waters when they were taken on Friday.

At a briefing in London, the Ministry of Defence said it "unambiguously contested" claims that the UK vessel had strayed into Iranian waters.

Prime Minister Tony Blair's spokesman said the evidence presented by the MoD was "difficult to dispute".

Earlier, Turkey's prime minister was quoted as saying his diplomats may be allowed to meet the group, taken in the Gulf.

According to CNN Turk television, Recep Tayyip Erdogan gave the indication after meeting the Iranian foreign minister and before an Arab leaders' summit in Riyadh, Saudi Arabia.

The Foreign Office said reports that Turkish diplomats may be granted access to the detainees should be treated with "caution".

On Tuesday, UK Foreign Secretary Margaret Beckett discussed the plight of the Britons with Turkey, its Nato ally.

The eight sailors and seven marines, based on HMS Cornwall, were taken at gunpoint in the northern Gulf as they returned after searching a boat.

Iran says they were trespassing in Iranian waters, but the UK has insisted the group were in Iraqi waters under a UN mandate.

Mrs Beckett cut short her visit to Turkey to return to the UK where she will and give a statement on the crisis to MPs.

She decided to return after a phone call to the Iranian foreign minister on Tuesday made no progress.

Downing Street has said that up to now the UK has been attempting to talk "discreetly" to the Iranians to secure the release of the Royal Navy personnel.

The prime minister's spokesman said that if the talks were unsuccessful, the government may have to become "more explicit" with Iran.

Salah Al Shaikhly, Iraq's ambassador to the UK, said his country would try to help the bid to secure the personnel's release and "the voice of reason should prevail".

BBC News has broadcast footage of an interview it carried out last week with Faye Turney, the only woman among the 15 who were captured

It showed the married 26-year-old, who had a photo of her three-year-old daughter very close at hand throughout the filming, on the Cornwall just hours before the incident.

The BBC has been told the group are being held at an Iranian Revolutionary Guards Corps base in Tehran, where they are said to be being treated humanely.

The Foreign Office says it has been unable to confirm reports by Iran's official news agency Irna that Mohammad Ali Hosseini, spokesman for Iran's Foreign Ministry, had said British diplomats would be given access to the group after the "primary investigation" by Iranian officials.

He was quoted by Irna as saying: "The media campaigns and provocative ... remarks regarding the violation of Iranian territorial waters by the British sailors are doing nothing to help settle the affair."