Al-Qaeda threatens attack on Brown
The message was posted in English to an al-Qaeda-affiliated online forum by "Umar Rabie al-Khalaila", US-based monitoring service the SITE Intelligence Group said.
The group, blamed for suicide bombings in London in 2005, vowed fresh attacks if Britain fails to withdraw its troops from Iraq and Afghanistan by the end of March.
"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 message said.
It vowed that suicide bombers would also "target all embassies, crusaders centre(s) and their interests throughout the country."
In July 2005, four suicide bombers killed themselves and 52 other people in attacks on London's public transport system while last June saw failed car bomb attacks in London and Glasgow.
The message also demanded that the British government "free all Muslim captives from Belmarsh prison" in south-east London which houses Britain's most high-profile Islamist militants.
SITE said the author of the message, "Umar Rabie al-Khalaila", is the same individual who posted a January 2 message claiming the establishment of al-Qaeda in Britain with the purpose of carrying out large scale attacks and killing political leadership.
It was not possible to independently verify the authenticity of the message, which was posted in an open section of the website, unlike "official messages from jihadist groups" which are usually posted in a section not open to the general members, SITE said.
In November last year, the head of Britain's domestic intelligence service MI5, Jonathan Evans, said that al-Qaeda was "grooming" children and young people to carry out attacks in the country.
The number of people with suspected links to terrorists rose from 1600 in 2006 to at least 2000 in 2007, he said.
MI5 said the terror threat level facing Britain is currently "severe", meaning there is a high likelihood of future attacks.