Fifteen Royal Navy personnel have been seized by Iranian naval warships in Iraqi territorial waters, the Ministry of Defence has said.

The incident happened during a "routine boarding operation" of a merchant ship off Iraq.

Foreign Secretary Margaret Beckett said she had asked Iran for "a full explanation" and Sir Peter Ricketts, Permanent Under-Secretary at the FCO, met with the Iranian ambassador in London, Rasoul Movahedian, to demand their immediate safe release.

In a statement the MoD said: "At approximately 10.30 Iraqi time this morning, 15 British naval personnel, engaged in routine boarding operations of merchant shipping in Iraqi territorial waters in support of UNSCR 1723 and the government of Iraq, were seized by Iranian naval vessels.

"The boarding party had completed a successful inspection of a merchant ship when they and their two boats were surrounded and escorted by Iranian vessels into Iranian territorial waters.

"We are urgently pursuing this matter with the Iranian authorities at the highest level and on the instructions of the Foreign Secretary, the Iranian ambassador has been summoned to the Foreign Office. The British Government is demanding the immediate and safe return of our people and equipment."

The personnel seized were from the Type 22 frigate HMS Cornwall.

The ship's commanding officer Commodore Nick Lambert said that every effort was being made to secure the release of the boarding party.

He said that contact had been lost with the group after they had carried out the search of the cargo boat.

Commodore Lambert said: "We did have a helicopter in the area. Our understanding from that is that the boarding party returned to its boats which were then promptly arrested by a group of Iranian patrol boats inside Iraqi territorial waters."

A journalist on board HMS Cornwall at the time of the incident said that they had just boarded a dhow.

He said: "While they were on board, a number of Iranian boats approached the waters in which they were operating - the Royal Navy are insistent that they were operating in Iraqi waters and not Iranian waters - and essentially captured the Royal Navy and Royal Marine personnel at gunpoint."

It is not the first time that British servicemen have been taken captive by Iranian forces in the troubled waters between Iran and Iraq.

Eight servicemen were seized and detained in July 2004 after their patrol boats were said to have strayed into the Iranian side of the Shatt al Arab waterway.

The capture of the six Royal Marines and two Royal Navy sailors sparked a tense stand-off.

The men were blindfolded and held for three days during which they were paraded on Iranian TV.

Sir Richard Dalton, who was the UK's ambassador to Iran at the time of the incident, said the current situation demanded "firm but patient diplomacy".

He said the taking of the sailors could be an effort to send a signal to the UK but that it was too early to be sure whether it had been ordered from the top.

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