Three British boats and their crews were seized in the Shatt al-Arab waterway, near the Iraqi border. Al-Alam television said the men admitted entering Iranian territory and were one kilometre over the border.

The UK government immediately demanded an explanation from Tehran. ‘We are trying to get the Iranians to explain exactly what they mean,’ a spokesman said. The Iranian authorities have not given British officials access to the men or told them where the men are being held.

Al-Alam quoted Iranian officials as saying that the men were to be prosecuted on charges of ‘illegally entering Iran’s waters’. Iranian interrogators have been questioning the British men, who the UK Defence Ministry says were part of a team training Iraqi police who were delivering a boat to the Iraqi Riverine Patrol Service when arrested. The UK government said the boats were unarmed and the sailors were carrying personal weapons only.

UK Foreign Secretary Jack Straw held a telephone conversation with his Iranian counterpart Kamal Kharazzi, although no information has yet been released as to the outcome of the conversation.

Relations between the UK government and Tehran are already difficult, given the submission of a draft resolution last week to the International Atomic Energy Agency (IAEA) by Britain, France and Germany, which criticised Iran over its lack of co-operation over nuclear activities.

There have also been angry demonstrations by Iranians outside the British embassy in Tehran who have protested the occupation of Iraq.