Three men arrested under terrorism legislation are being questioned over internet threats to kill Gordon Brown and Tony Blair.

One of the men being questioned by police in Manchester is suspected of posting a death threat on a recognised jihadi website and styling himself Sheikh Umar Rabie al-Khalaila, leader of al-Qaeda in Britain.

The threats appeared on the al-Ekhlaas website – which has been used by al-Qaeda and is monitored by intelligence agencies - in January.

They gave warning of a wave of suicide attacks in Britain and against British interests around the world unless the group’s demands for a withdrawal from Iraq and the release of Muslim prisoners were met.

Detectives investigating the background to the posting arrested the three men last week in an operation run by Greater Manchester Police’s counter-terrorism unit.

The detainees are aged 21, 22 and 23 and all come from Blackburn, Lancashire, where extensive searches have been carried out.

One man was detained in the Accrington area and the other two were arrested at Manchester airport before they could board a flight to Helsinki.

The Times understands that British police are concerned about the potential link to similar groups in Scandina-via, and travelled to Finland this week to carry out further inquiries.

There is no known history of Islam-ic extremism in Finland but there has been an upsurge in Denmark in recent years.

Police were granted further warrants of detention to continue questioning the men until next Thursday, when they may seek to extend that period again.

The internet threats were reported by The Times earlier this year and the full text of the posting, which originally appeared in Arabic, has since been obtained.

It was headed, “Statement of the Leader of al-Qaeda in Britain, Sheikh Umar Rabie al-Khalaila” and began by offering “a truce to the British government”.

The author of the document demanded “a complete withdrawal of British troops from Afghanistan and Iraq” and the release of “all Muslim captives from Belmarsh Prison”.

It specifically named the extremist clerics Abu Qatada and Abu Hamza al-Masri as two inmates who should be freed. The courts have since ordered the release on bail of Abu Qatada, who is now living under a 22-hour curfew in West London.

The statement added: “If the British Government fails to respond to our demands by the last day of March 2008, as they fail to answer to the truce of our Sheikh Osama bin Laden . . . then the Martyrdom seekers of the organisation of al-Qaeda in Britain will target all the political leaders, especially Tony Blair and Gordon Brown. We will also target all Embassies, Crusader Centres and their interests throughout the country, with the help of Allah.”

The author signed off: “Umar Rabie al-Khalaila. The leader of al-Qaeda in Britain.”

Sources said that the arrests did not appear to be connected to any imminent terrorist attack but there was concern about the propagation of violent and extremist ideology.

Reporting on the posting in January, the Jamestown Foundation in Washington DC said: “British security officials had difficulty assessing the seriousness of the threat, which appeared only briefly on al-Ekhlaas.”

The threats had, however, coincided with warnings from Spanish police about a group of Pakistani nationals en route from Barcelona to London.

Those fears led to the arrest of a number of men at Heathrow, but they were able to demonstrate that they had no connection to terrorism.

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”.

Security agencies are increasingly concerned about the use of the internet by jihadi groups to radicalise and recruit young people and to instruct them in terrorist techniques.

A large number of websites are monitored by intelligence organisations.

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