A fourth man has been arrested over alleged threats to kill British Prime Minister Gordon Brown, police sources said Tuesday.

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

He is the fourth member of a suspected terror cell held by detectives in the past two weeks.

Two men were arrested at Manchester Airport on Aug. 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 in their 20s.

Police have until Thursday to charge, release or seek an extension to the trio's custody.

The latest arrest came after a dawn raid on an apartment behind a Halal butcher's shop, where

A police source said officers were investigating threats to kill Brown.

It is understood no concrete plot was in place but that officers discovered a written threat on an extremist jihadi Web site earlier this year.

local residents said the man — who is said to be white — lived alone.

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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