An "important" suspect in the London attacks is under arrest, a Pakistani official said on Wednesday, as investigators used telephone numbers provided by Britain to determine who may have had contact with the bombers.

"We have an important man in our custody," a senior intelligence official said on condition of anonymity. "He has some direct links with the London attacks."

The official, speaking in the eastern city of Lahore, would not name the man, say when he was arrested or elaborate on his alleged links with the 7 July London bombings that killed 56 people and injured 700 on three underground trains and a double-decker bus.

The official did not want to be identified because he is not authorised to speak to the media and because of the secretive nature of his job.

"We have apprehended several suspects," said Lahore Police Chief Tariq Salim.

The London blasts killed 56 people

"More arrests are in the offing." Many hardline groups operate clandestine offices in Lahore, which is near the Indian border.

Investigators are trying to determine whether the British-born bombers, three of whom are of Pakistani origin and travelled to Pakistan last year, received training or other help from hardliners in Pakistan.

Investigation 

Two intelligence officials in the capital, Islamabad, said on Wednesday that British authorities had asked their Pakistani counterparts to check about 100 Pakistani telephone numbers for possible links to the bombers.

Authorities had concluded that nearly 80 numbers did not provide information useful to the case, said the senior official.

The remaining 20 numbers, which included fixed lines and cellular telephones, were under investigation, the official said.

Some were no longer in use, and authorities were trying to establish who had originally registered them.

The officials said Britain also provided names of several people in Pakistan who allegedly received calls from the bombers over the past year.