AL QAIDA terrorists based in Afghanistan may have been behind an alleged plot to bring down airliners over US cities, it was claimed last night.

Meanwhile, detectives investigating the alleged plot were granted warrants for the further detention of 22 people held in the operation.

A total of 24 people were arrested following raids in London, High Wycombe, in Buckinghamshire, and Birmingham were carried out by police in the early hours of Thursday.

A Metropolitan Police spokesman said the warrants allowed officers to detain the 22 suspects until Wednesday.

A hearing regarding the detention of another person was adjourned until Monday. Another of the people arrested was released with no further action.

Nineteen of the 24 people arrested over the alleged plot, at least three of whom are converts to Islam, have had their assets frozen by the Bank of England.

Meanwhile, the worldwide intelligence operation into the alleged plot to crash ten jets over US cities continued.

In Pakistan, authorities said seven people had been held in Karachi and Lahore.

They claimed it was the arrests there that had triggered the police operation to act.

Its foreign ministry also said there were indications of an "Afghanistan-based al Qaida connection" to the alleged plot to blow up transatlantic passenger jets in mid-flight.

One of the men held, Rashid Rauf, had links with al Qaida, according to Pakistani Interior Minister Aftab Khan Sherapo.

The FBI and MI5 were working flat out to identify whether or not a second al Qaida cell may be waiting to launch attacks in the event of the first alleged plot being discovered.

Intelligence sources in the US claimed the huge surveillance operation began shortly after the July 7 London Underground bombings last year, when Scotland Yard received a call from a worried member of the Muslim community.

By late last year, the operation involved several hundred intelligence officials on three continents after MI5 decided to confide in Pakistani and US intelligence services.

They kept tabs on dozens of people and watched as the alleged conspirators made trips from Britain to Pakistan to raise money and refine the plan.

Investigators believe the operation involved three cells. Members of each group may not have been aware of the others or the size of the alleged operation, which rivalled the September 11 hijackings in scope.

The disruption caused by the worldwide terror alert continued to cause chaos last night.

Although flights from Newcastle and Durham Tees Valley were getting back to normal, long-distance travellers were warned to expect problems at major international airports.

In the North-East, many passengers found themselves trapped in the chaos which has engulfed Heathrow and Gatwick since Thursday morning.

No flights into or out of Teesside were cancelled yesterday, and delays were substantially shorter than the previous day.

Newcastle Airport suffered many more problems yesterday, with 22 flights into and out of the airport cancelled.

EasyJet had 12 flights cancelled and British Airways ten. All were internal flights, plus the budget airline's flights to and from Budapest.

The situation appeared to be easing, but last night eight flights into and out of Newcastle over the weekend - to Malaga, Geneva, Alicante and Heathrow - were cancelled