Travel chaos grips UK's airports
Passengers are being asked to be patient when facing delays
BAA advice
Heathrow Airport has been closed to all incoming flights that are not already in the air, following a police anti-terror operation.
Stringent security measures have been put in place at airports across the UK and passengers face severe delays.
German carrier Lufthansa, Spanish airline Iberia and Olympic have all cancelled flights to the UK.
And BAA has urged passengers planning to fly out of Heathrow to postpone plans unless the journey is essential.
It has said that all incoming short-haul flights to Heathrow had been cancelled for the time being.
Air France said it had cancelled five flights from Heathrow to Paris.
BBC News website's Krishan Ramakrishnan at Heathrow said travellers at the airport were not as worried by the security operation as by the inconvenience of just taking passports and medicines on the plane.
All cabin baggage must be processed as hold baggage and carried in the hold of passenger aircraft departing UK airports
Courteney Dane, who is travelling back to Australia, said: "I'm worried about the cost and the fact I've got a 24 hour flight with nothing to read. I'm not really worried about the security, it's just a nuisance."
Our reporter said there was a massive queue to get into the Terminal Four building.
He said many travellers were working out ways to travel to Europe by alternative methods, such as Eurostar.
He also described Terminal Three as a "sea of people", as a huge queue built to get in to the departure lounge.
Charlotte Demant, a make up artist, has been in Ghana for two weeks and was on her way home to Denmark.
She said: "I can understand why they are worried about security but the airline has not been helpful.
"We should have at least been given something to eat and drink and money to phone home. I'm going to end up spending the night here in the cold airport."
Scotland's airports are busy but passengers are still being advised to turn up even though there may be long queues for check-in.
There were extra security checks at Glasgow airport and Edinburgh, Aberdeen, Prestwick and Dundee were experiencing delays.
At a glance: disruption
West Midlands disruption
Bristol airport delays
Disruption in Yorkshire
Manchester Airport has said there are delays on all flights of between one and three hours.
The only flights from the airport cancelled were those BMI and BA flights destined for Heathrow.
Stansted airport is open, with flights arriving and departing but passengers are experiencing waits of up to three hours to get through checks.
Extra police have been drafted in to many UK airports.
At Gatwick, airport worker Warren Tarling said he had never seen the airport so busy.
"There's thousands upon thousands of people. All flights out of the airport have been delayed," he said.
"Most people have been polite but some have been angry. Generally people have been understanding because of the nature of the problem."
Donald Morrison, a spokesman for BAA at Glasgow Airport, said there would be delays at security for all passengers.
"We are bringing in additional security staff as we speak and we are hoping that passengers will be able to pass through security safely and swiftly but we would advise passengers to give themselves extra time when they come to the airport."
Transport Secretary Douglas Alexander said all airports and airlines operating into and out of the UK would be on a heightened state of security.
"Maximum security unfortunately will involve immediate and severe disruptions for passengers with significant delays likely at all UK airports," he said.
"What these changes mean in practice is that all hand baggage will now have to be checked in with only a small number of essential items allowed through search controls.
"Exceptions will be in place for those travelling with infants and for prescription medicines."
Passengers are being asked to check in most items of baggage, apart from wallets and purses, identity and travel documents, prescription medicines, spectacles and sunglasses and keys.
Those travelling with children will be allowed items essential for caring for them on the flight.
Passengers wanting detailed information are being advised to contact the airline they were due to travel with. Information would also be available on the Home Office, the Department for Transport and Foreign Office websites.
