Terrorists are plotting to blow up five European passenger jets in a Christmas "spectacular", security experts say.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. "Everyone is expecting something catastrophic very soon".Threats of bombs hidden in toothpaste tubes caused huge disruption to travellers to the Sochi Winter Olympics and stringent security checks at UK airports will cause huge queues during what is the busiest time of the year for airlines. Security experts also warn that future terror attacks could be carried out by sleepers - western converts as likely to have blond hair and blue eyes as dark skin and beards. 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. Last night leading terrorism expert Dr Sally Leivesley said the banning of hand luggage and hand held electronic devices is unlikely to be accepted by the airlines despite the risks.She explained: "Efforts to ban all electronic devices including computers from business class passengers which reaps huge revenue for the airlines is nearly unsustainable in their business model. "However, I would contend that even a mobile phone in the hold is capable of being used as a trigger for a bomb or used as a bomb in its own right. "The most likely scenario is a plane heading towards a major European capital. "There is likely to have been dry runs already with terrorists testing airport security." Dr. Leivesley, a former Home Office risk adviser, said that terrorists are now more likely to be "white, blond and blue eyed" who are radicalized in as little as five weeks. She also claimed that female terrorists were becoming a significant risk, adding: "Crime profiling shows that white, middle class women, who are better than averagely educated, are susceptible to the terrorist narrative. "They see themselves at the forefront of attempts to change the world and are represent a very dangerous tool for the terrorists. "These sleepers will have been from ordinary and not very religious families and not only is the threat from them here but also when they return battle hardened from Syria and Iraq."In a speech earlier this week, Home Secretary Theresa May warned that numerous Islamic militants were looking to attack the UK. She said: "Now the threat is more diverse in terms of the number of groups out there who will be looking to carry out attacks in the West. "It's also more diverse in that not everybody's affiliated to a particular group - some are self-starting groups and you do have individuals, perhaps the "lone wolves" or the volatile individuals. 'We have to look across the board at all sorts of threats that are out there." Scotland Yard would not comment on the claims. A spokeswoman for the Department for Transport, which looks after air security, said she could not comment on the specific allegations. But she added: "We keep airport security under constant review."