British police have arrested a man in northwest England on suspicion of preparing acts of terrorism and were searching five properties as part of their investigation.

"It is absolutely vital to stress - and I hope at the same time reassure the public - that there is no current threat or evidence of an imminent attack," said Detective Chief Superintendent Tony Mole of the North West Counter Terrorism Unit (NWCTU).

"We will of course keep the public informed as things progress and would once again encourage them not to be unduly worried."

The NWCTU said the man, 31, was in police custody after he was arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.

Searches at the five premises in St Helens and Liverpool would last several hours and possibly days, police said.

Britain raised its international terrorism threat level to "severe" last August, meaning an attack is considered highly likely, and there have been dozens of arrests since then.