A 25-year-old man was being questioned today over threats to kill Gordon Brown and Tony Blair after he was arrested on terrorism charges.

The man, who is believed to be white, is the fourth resident of Blackburn to be arrested this month in connection with posting a death threat on a recognised jihadi website.

He was arrested at 6.40am this morning. Specialist officers were continuing to carry out searches this morning in the Whalley Range area of the town, where a police van was seen parked outside an Islamic bookshop.

Three men, aged 21, 22 and 23, were arrested on August 14. Two were held at Manchester Airport as they were about to board a flight to Finland, and the third man was detained in Accrington, Lancashire.

One of the men is suspected of posting a death threat on a known extremist website and styling himself as Sheikh Umar Rabie al-Khalaila, leader of al-Qaeda in Britain.

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

No terror plot was believed to be in place, but there were calls for the death of the Prime Minister on the al-Ekhlaas website - which has been used by al-Qaeda and is monitored by intelligence agencies - in January.

The message gave warning of a wave of suicide attacks in Britain and against UK interests around the world unless the group's demands for a withdrawal from Iraq and the release of Muslim prisoners were met.

After the latest arrest, police Community Support Officers were handing out leaflets explaining to people in the residential area of Whalley Range that anti-terror police would carry out their searches as quickly and calmly as possible today.

A Lancashire police spokeswoman 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.

“We will endeavour to carry out any searches as quickly as possible to minimise impact on the area concerned. However, inquiries of this nature are complex and may take time to resolve.”

Two of the arrests earlier this month were made as the suspects made their way to Finland. It is understood that British police are concerned about the potential link to terrorist groups in Scandinavia, and officers travelled to Finland this month to carry out further inquiries.

There is no known history of Islamic extremism in Finland, but there has been an upsurge in Denmark in recent years.

The death threat, thought to have been posted by British-based extremists in January, was headed: “Statement of the Leader of al-Qaeda in Britain, Sheikh Umar Rabie al-Khalaila,” and began by offering “a truce to the British government”.

The author of the document demanded “a complete withdrawal of British troops from Afghanistan and Iraq” and the release of “all Muslim captives from Belmarsh Prison”.

It specifically named Abu Qatada and Abu Hamza al-Masri, the extremist clerics, as two inmates who should be freed. The courts have since ordered the release on bail of Abu Qatada, who is now living under a 22-hour curfew in West London.

The four arrests did not appear to be connected to any imminent terrorist attack but police are concerned about the propagation of violent and extremist ideology.

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