A fourth man has been arrested over an alleged plot to kill the Prime Minister, police sources have said.

The 24-year-old was held in Blackburn, Lancashire, under the Terrorism Act 2006.

He is the fourth member of a suspected terror cell to be arrested by detectives in the past fortnight.

A police source said officers are investigating threats to kill Gordon Brown.

It is understood no concrete plot was in place but officers discovered a written threat on an extremist Islamic website earlier this year.

Two men were arrested at Manchester Airport on August 14 as they prepared to board a flight to Finland.

A third man was held in Accrington, Lancashire, on the same day. All three are aged in their 20s.

Police have until Thursday to charge the three, release them or seek an extension to their custody.

The arrest came after a dawn raid on a flat behind a Halal butcher's, where local residents said the man - who is said to be white - lived alone.

Specialist counter-terrorism officers are examining the address in Whalley Range, north of the town centre and a police van was seen parked outside a nearby Islamic bookshop.

Whalley Range is a quiet residential street a short walk from Percival Street, where officers searched a house after the earlier arrests.

Police Community Support Officers were distributing leaflets in the area informing residents about the anti-terror operation.

Chief Superintendent Andy Rhodes, from Lancashire Police, said: "This is a very sensitive time for all our communities and we are working hard to ensure everyone is kept up to date with as much information as we can give."

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