An unofficial police source said the recordings . . . discovered on laptop computers . . . appear to have been made by some of the suspects being questioned.

Scotland Yard has refused to comment on what officers are finding.

Police are continuing to search woodland at High Wycombe in Buckinghamshire and 14 addresses.

The Metropolitan Police, which is leading the inquiry, has said it has already completed searches of 36 business and residential sites. Twenty vehicles have also been searched.

Meanwhile, it has emerged that every police force in the UK is now involved in the investigation.

The 43 forces in England and Wales, eight in Scotland and the Police Service of Northern Ireland have sent, or are sending, officers to assist.

Hundreds are said to be involved with further officers on patrol at airports.

A spokeswoman for the Association of Chief Police Officers said: "All forces across the UK are providing officers to help with the operation by the Metropolitan Police."

The alert began on 10 August after raids in London, High Wycombe and Birmingham.

A district judge has granted anti-terror officers an extra seven days to question 21 of the 23 people being held, and an extra five days to question the other two. Under the 2006 Terrorism Act the maximum period someone suspected of terrorist activity can be held without charge is 28 days, subject to regular court approval.

It is believed up to 17 people are being held in Pakistan over the alleged plot . . . with two British nationals of Pakistani descent among them.

Scotland Yard officers have also flown to Pakistan to liaise with the authorities there over the questioning of Rashid Rauf, the only man who has so far been named by Pakistani authorities.

SEE ALSO MICHAEL O'LEARY AND THE TERROR THREAT, PAGES 14-15