We've been extremely lucky since 9/11 in avoiding a major, mass-casualty terror attack.  Some foiled plots we know about; others remain secret.  But it's been an accepted fact that, out of 100 plots, the terrorists can be stymied in 99 attempts and have to be successful only once to cause a tragedy.

This should be kept in mind when reading this chilling report by the Sunday Express that reveals a plot uncovered by the British government to blow up five European passenger planes around Christmas.  The threat has been taken so seriously that thought has been given to banning all carry-on luggage, as well as all electronic devices.

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.

"Everyone is expecting something catastrophic very soon".

The U.S. heightened security at some international airports over the summer, although it is unclear if this plot is connected to the U.K. intel.

The federal government is increasing security measures at some international airports with direct flights to the United States, based on recent intelligence about the continuing, worldwide terror threat, Homeland Security Secretary Jeh Johnson said Wednesday.

Johnson gave no information about a specific threat, saying only that the change was based on the U.S. and its allies “continually (assessing) the global threat environment.”

However, the beefed up security is almost certainly a response to recent intelligence reports suggesting that Al Qaeda-linked terrorists in Syria are working with members of Al Qaeda in the Arabian Peninsula to blow up a commercial aircraft headed to the U.S. or Europe, as reported first by ABC News.

A counterterrorism official told AP that the terrorists are attempting to create a bomb that would go undetected through airport security.

The official declined to describe the kind of information that triggered this warning. Officials in the past have raised concerns about non-metallic explosives being surgically implanted inside a traveler's body, designed to be undetectable in pat-downs or metal detectors.

The Sunday Express report seems to acknowledge that the information uncovered by U.S. intelligence may have been confirmed by Great Britain:

The latest threat is understood to have been uncovered by American intelligence officers who fear that despite success in taking out key Al Qaeda bomb-makers, the genie is out of the bottle and sleeper cells are currently preparing for a doomsday scenario.

David Drugeon, a 24-year-old Frenchman and one of Al Qaeda's most trusted bomb-makers was killed by a cruise missile strike earlier this month to the west of the Alepo, Syria.

Drugeon, who was part of an Al Qaeda off-shoot called the the Khorasan group, and was said to be targeting US and UK airlines with non-metallic explosive devices, which could be concealed in mobile phones, computers and printer cartridges.

Although now dead his work remains a huge threat as his skill set is likely to have been spread among a number of terror apprentices, including an unidentified Saudi-born bombmaker.

If terrorists have been able to construct a non-metallic bomb, it will enormously complicate security procedures.  There are still countermeasures that can be used, but machines that can sniff out explosives are still not widely distributed at major airports.

Have the terrorists gotten the drop on us?  Just a few weeks ago, Homeland Security chief Jeh Johnson admitted that all of our efforts in Syria to disrupt al-Qaeda and ISIS have gone for naught.  They are continuing their plots and may have decided to carry them out regardless of their chances for success.

If so, it may be a very bloody Christmas in Europe.