'Plot to blow up planes' foiled
A terrorist plot to blow up planes in mid-flight from the UK to the US has been disrupted, Scotland Yard has said.
It is thought the plan was to detonate explosive devices smuggled on aircraft in hand luggage.
Police have arrested about 18 people in the London area after an anti-terrorist operation lasting several months.
Security at all airports in the UK has been tightened and delays are expected. MI5 has raised the UK threat level to critical - the highest possible.
I was handed a piece of paper saying that pretty much nothing could be taken on board the plane
BBC journalist at Gatwick
According to MI5's website, critical threat level means "an attack is expected imminently and indicates an extremely high level of threat to the UK".
Scotland Yard said in a statement that their investigation into the alleged plot was a "major operation" which would be "lengthy and complex".
"We would like to reassure the public that this operation was carried out with public safety uppermost in our minds."
Home Secretary John Reid is due to make a statement early on Thursday morning.
The Department for Transport set out the details of the security measures at UK airports.
The current threat level is assessed as critical as of 10th August 2006 - this means that an attack is expected imminently and indicates an extremely high level of threat to the UK
Passengers will not be allowed to take any hand luggage on to any flights in the UK, the department said.
Only the barest essentials - including passports and wallets - will be allowed to be carried on board in transparent plastic bags.
"We hope that these measures, which are being kept under review by the government, will need to be in place for a limited period only," the statement said.
BBC journalist Joe Lynam encountered the increased security measures at Gatwick airport.
"I was handed a piece of paper saying that pretty much nothing could be taken on board the plane," he said.
"Everything had to be checked in and that includes mobile phones, ipods, wallets - even spectacle cases had to be checked in."
David Learmount from Flight International Magazine said he expected passengers to be searched much more carefully.
He added: "This is the first time this measure has actually been taken. Certainly I've never seen hand luggage banned."