Postings to extremist websites were attributed to group calling itself al-Qaida in Britain

Detectives investigating internet threats to kill Gordon Brown and Tony Blair today arrested a fifth man.

The 29-year-old was detained in Derby. Four other men have been arrested in the last fortnight. All are aged in their 20s and are being held under the Terrorism Act 2006.

The threats were posted on an extremist website in January and were attributed to a previously unknown group calling itself al-Qaida in Britain.

A police spokeswoman said: "The arrest is part of an ongoing investigation by Lancashire constabulary and Greater Manchester counter-terrorism unit."

Thirteen days ago police detained the first three men, aged 21, 22 and 23. Two were held at Manchester airport as they were about to board a flight to Finland. The third was arrested in Accrington, Lancashire.

Detectives have until tomorrow to charge or release them, or seek an extension to their custody.

The fourth man, 24, was arrested in Blackburn in a dawn raid yesterday. He is believed to be a white Muslim convert who lived alone.

Police said no bomb-making or other terrorist equipment had been recovered and there was no apparent evidence of a plan to carry out any threats.

Counter-terrorism officers were yesterday examining an address in Whalley Range, north of Blackburn town centre.

Police community support officers were distributing leaflets in the area explaining the operation to residents and seeking to reassure the public.