London - A fifth suspect has been arrested as part of investigations into a threat to kill British Prime Minister Gordon Brown, police said Wednesday.

A 29-year-old man was arrested in Derby, police said. Three men were arrested about two weeks ago in Manchester and a fourth on Tuesday in Blackburn. They were all aged between 20 and 30.

Police were searching an address in Derby.

Reports said that the threat against Brown appeared on an Islamist website in January and was signed "al-Qaeda in Britain." Brown's predecessor Tony Blair was also threatened.

No concrete plot existed and no explosives or other terrorist material has been uncovered, the Guardian newspaper quoted police as saying.