Al-Qaeda has threatened a wave of suicide bombings in Britain unless all troops are withdrawn from Iraq and Afghanistan and Islamist prisoners are freed from Belmarsh jail by the end of March.

The statement, which also includes specific assassination threats against Gordon Brown and Tony Blair, appeared earlier this week on al-ekhlaas.net, a recognised jihadi website.

It was posted in English under the banner of "Al Qaeda in Britain" but remained on the site for only a brief period.

Earlier this month the website, which is monitored by intelligence services around the world, announced the formation of al-Qaeda in Britain under the leadership of someone calling himself Sheikh Umar Rabie al-Khalaila.

The same individual signed the second statement, which appeared on January 24 and began with the demand for the withdrawal of British forces from Iraq and Afghanistan.

The message continued by insisting on the release of "all Muslim captives" from Belmarsh high-security prison in south-east London. It named the radical clerics Abu Qatada and Abu Hamza al-Masri as prisoners who should be released immediately.

Abu Qatada, a Palestinian cleric who is fighting deportation to Jordan, is in the prison estate but is not held at Belmarsh.

Abu Hamza, the hook handed former imam of Finsbury Park mosque who is awaiting extradition to the United States, is a key figure among the terrorist prisoners at Belmarsh.

The al-Qaeda statement added: "If the British government fails to respond to our demands within the last day of March 2008. . . then the martyrdom seekers of the Organisation of Al-Qaeda in Britain will target all the political leaders, especially Tony Blair and Gordan (sic) Brown."

The list of targets would also include "all embassies, crusader centres and their interests throughout the country".

The SITE Intelligence Group, which monitors jihadi internet activity, said it was not possible to verify the authenticity of the statement. But it noted that it had been posted on an open section of the website and not one of the more secure closed forums normally used by al-Qaeda affiliates.

Reporting on the posting, the respected Jamestown Foundation in Washington DC, said: "British security officials had difficulty assessing the seriousness of the threat, which appeared only briefly on al-ekhlaas, but comes at a time of heightened vigilance following warnings from Spanish police that Pakistani nationals intended to launch suicide attacks on the London transit system."

Scotland Yard's Counter Terrorism Command is aware of the al-ekhlaas site but refused to say whether it was being monitored or whether the latest threat had any credibility.

A spokesman said: "We regularly monitor a range of websites and organisations. Any information we receive will be considered and appropriate action taken."

The current threat level in Britain is "severe" meaning, according to MI5, that there is "a high likelihood of future terrorist attacks" and "a continuing high level of threat to the UK".

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