The threat has been taken so seriously it came close to leading to an outright ban on all hand luggage, a senior insider has revealed.

Mobile phones and electronic devices could still be banned from plane cabins, with the threat of a 9/11-style coordinated attack on London and other major cities feared imminent.

The warning comes as Whitehall officials admit that a terror strike on the UK is now “almost inevitable” particularly with British jihadis returning from fighting alongside the Islamic State in Iraq and Syria.

An airport security source told the Sunday Express: “We’ve been told that five planes are being targeted in a high profile hit before Christmas. They’ve been waiting for the big one.

“We have many scares but this one nearly got hand baggage pulled from all airlines. The threat is still alive and real.”

The plot, which has been known about for the past two months, is thought to involve Islamists smuggling bombs on to planes bound for major European destinations before Christmas.

The well-placed security insider said talks had been held about how to deal with it, with suggestions made to ban hand luggage.

The source said: “High level negotiations are continuing at Governmental level but at the moment there has been little done to respond.

“There is paralysis because of the difficulty of banning hand luggage which is one of the strongest weapons we have against the new threats.

“All electronics may be banned from hand luggage and placed in the hold, that has been considered, and there has been behaviour analysis training at airports but while it’s effective, it’s difficult to roll out quickly and is not a sufficient safety net.

“The threat is aimed at Europe. The U.S has improved their security over the summer but we have not.