TERRORISTS plotting to blow up passenger jets in mid-air could have been only days away from launching their mission to commit "mass murder on an unimaginable scale", it was revealed last night.

Their plan was to detonate liquid explosive devices smuggled in hand luggage on as many as ten aircraft leaving UK airports for the US.

Officials in the US said the plan - described as a terrorist "spectacular" on a par with the 9/11 atrocity if it had been successful - bore some of the hallmarks of an al Qaida plot.

A joint investigation by police and MI5 reached a critical point on Wednesday night, and anti-terror officers swooped in the early hours of yesterday, arresting 24 young Britons of Pakistani origin - the youngest of whom was reported to be 17 - in London, the Thames Valley and the West Midlands.

Last night, Pakistani government officials said their intelligence agencies had helped British security officers crack the terror plot, and several people had been arrested.

Home Secretary John Reid said that if the plot had succeeded, it would have caused death on an "unprecedented scale".

There were claims last night that the terrorists could have been planning to bring down airliners over major cities, either in the UK or US.

It prompted the British authorities to raise the terrorist threat level to critical for the first time.

US President George Bush said the plot was "a stark reminder that this nation is at war with Islamic fascists who will use any means to destroy those of us who love freedom".

Scotland Yard said it was designed to cause "untold death and destruction".

In response to the threat, stringent security measures were imposed at all UK airports early yesterday, causing widespread chaos. A ban on hand luggage along with other checks and precautions led to huge queues at airports where scores of flights were cancelled.

Details of the plot emerged for the first time in the early hours as the nation woke to the news.

But police later revealed the arrests followed an "unprecedented level of surveillance" - stretching back to December last year - involving meetings, movements, travel, spending and the "aspirations of a large group of people".

The arrests were made after a series of raids in London, Birmingham and High Wycombe, Buckinghamshire. Precise locations of the swoops in the capital were last night largely unknown, but one was known to have taken place in Walthamstow, east London.

Thousands of travellers had their plans disrupted, but those at Durham Tees Valley Airport were largely unaffected.

A ban on all short-haul flights into Heathrow was lifted last night, but the airport was still not operating at normal capacity because many domestic flights remained cancelled.

Outbound flights were subject to severe delays because all passengers were being hand-searched.

All easyJet flights from London Stansted were cancelled, while Ryanair said it grounded a large number of services from London airports, with Stansted particularly affected.

Lucy Collinson, 21, and Jade Sanders, 24, were among those due to fly with easyJet from Stansted to Ibiza last night and were caught up in the chaos.

The friends, from Hartlepool, had travelled to London by coach from Darlington yesterday morning, but discovered their flight was grounded when they arrived at the airport.

Armed police had blocked off the drop-off point and passengers were being moved into the short-stay car park so officers could control the terminal.

Childcare worker Miss Collinson spent last night in the foyer of a hotel near the airport, with about 100 other stranded holiday-makers.

She will either head back to the North-East today or take a short break in London.

Last night she said: "I'm absolutely gutted.

"I know this is no one's fault, but it's left us heavily out of pocket.

"It's sod's law that this should happen on the day I was due to go on holiday."

A spokesman for the airport - one of the worst hit in the country - said: "We have had unprecedented problems because of what has happened. This is new territory for us.

"But there have been no significant problems with passengers being impatient or upset."