A suitcase containing items which could be used to construct a bomb was found by officers investigating an alleged plot to blow up airliners, it was reported last night.

The case was found by officers searching land called King's Wood in High Wycombe and a police source was quoted as saying the case contained "everything you would need to make an improvised device".

But other officers only confirmed that a suitcase had been found and Scotland Yard made no official comment.

The news came as officials claimed a key suspect in the alleged airliner bomb plot had links with an outlawed Pakistani militant group and met al Qaida figures inside Pakistan in the lead-up to his arrest.

As anti-terror detectives continued to question the 23 suspects held in the UK, intelligence officials in Pakistan disclosed further details of Rashid Rauf's alleged connections to the al Qaida terror network.

The arrest last week in Pakistan of Rauf, a British national and the brother of Birmingham man Tayib Rauf, one of the men detained here, is widely believed to have triggered the police operation to smash the alleged plot.

Officials claimed he had been in contact, through intermediaries, with a high ranking al Qaida leader at large in neighbouring Afghanistan, and that he had met al Qaida figures inside Pakistan.

He also had links to the militant group Jaish-eMohammed and was related by marriage to its leader, it was alleged.

Detectives have been granted more time to question the 23 suspects being held in London over the alleged plot. They have another week to continue questioning 21 of them, and another five days to question the other two.

However, the complexity of the case means they are likely to continue applying for a series of further extensions - taking them close to limit of 28 days - before deciding whether to charge or release the suspects.

A person arrested in the Thames Valley area on Tuesday was released without charge on Wednesday night.

Officers from the force have also flown to Pakistan to liaise with the authorities there over the questioning of Rauf. It is still unclear whether the Government has made a formal request for his extradition.

A number of other people are being held in Pakistan over the alleged plot.

One intelligence official there has claimed that the suspects were not "experienced" and "trained" like al Qaida operatives who had carried out the September 11 attacks or last year's London bombings, but were "filled with hate" for Britain and the United States.

"I don't know how close they were from executing the attacks, but I personally believe that they wanted to do it to mark the (fifth anniversary of) 9/11 attacks," the official said. "I personally think they would have carried out the attacks if they had been experienced enough."

The suspects had not attended terror-training camps in Pakistan or neighbouring Afghanistan and had relied on information gleaned from text books on how to make bombs, officials alleged.

The alleged plot was probably sanctioned by al Qaida's deputy leader Ayman alZawahri, a senior Pakistani intelligence official claimed.

Searches were continuing at a number of residential and business premises across London, the West Midlands and Thames Valley.

The most high profile of those is at King's Wood in High Wycombe, Buckinghamshire. The alleged plot was to blow up US-bound transatlantic passenger jets in mid flight using liquid-based explosive devices hidden in hand luggage.