He 24-year-old was arrested yesterday morning in Blackburn, Lancashire, under the Terrorism Act 2006.

He is the fourth member of a suspected terror cell from the town held by detectives in the past fortnight.

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

It is understood no actual plot was in place but that officers discovered a written threat on an extremist jihadi website earlier this year. Two Asian men in the Whalley Range area of Blackburn said the man was once married.

Councillor Iftakhar Hussain added: "I don't know him personally. Someone told me he's been arrested.

"He's a white male, he's dressed in the kind of Muslim wear.He's a Muslim convert but he doesn't attend a local mosque."

Yesterday's arrest came after a dawn raid on a flat behind a halal butcher's shop where locals said the man lived alone.

Specialist counter-terrorism officers spent the day examining the flat.

The fourth terror arrest came 12 days after police arrested three men, aged 21, 22 and 23. Two were arrested at Manchester Airport as they were about to board a flight to Finland.

A third man was held in Accrington, Lancashire.

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

Whalley Range is a quiet residential street only 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 explaining the anti-terror operation and seeking to reassure the public.

Abdul Qureshi, chairman of the Lancashire Council of Mosques, said: "The police have been quite OK.

"They kept the community informed and they looked after the families of people taken into custody."

Mohammed Shafiq, chief executive of the Ramadhan Foundation, said: "The community has to wake up to the reality that terrorism is a real threat.

"People in Lancashire have to come forward and provide any evidence."

A police spokeswoman said: "The arrest is part of an ongoing investigation by Lancashire Constabulary and Greater Manchester Counter Terrorism Unit."