New security procedures have led to a fourth day of disruption to flights at the UK's busiest airports.
The new security measures are leading to delays
Ryanair has cancelled more than 30 flights from Stansted and British Airways cancelled a third of Heathrow flights and more than 20 from Gatwick.
BA has criticised operator BAA, which owns all three airports, calling on them to increase resources.
But BAA said these were "unprecedented times" and "the safety and security of the public is the utmost priority".
Tony Douglas, BAA's chief executive officer for Heathrow, said: "We are all responding to a national security threat.
"Given that this is the busiest time of the year, I think we should actually commend the work of the airlines, their staff and the operational staff for doing a really first-class job."
CABIN LUGGAGE ALLOWED
Pocket-sized wallets/ purses, plus contents
Passports/ travel tickets
Prescription medicines, not in liquid form unless verified as authentic
Essential medical items, eg diabetic kit
Glasses & sunglasses, no cases
Contact lens holders, no solution
Baby food & milk for those with infants - bottle contents must be tasted by accompanying passenger
Essential sanitary items for infants
Female sanitary items, unboxed
Tissues, unboxed, or handkerchiefs
Keys, but no electric key fobs
Deputy Prime Minister John Prescott, meanwhile, visited Stansted airport to see how the new security measures were being put into practice.
"Airport staff are working extremely hard in difficult circumstances, and are doing an excellent job," Mr Prescott said.
"Those staff I met today told me of their determination to continue to make air travel as safe as possible.
"Everyone's priority is safety, and the passengers I spoke to showed great understanding of why these security measures were put in place.
"As usual, the British public are showing great resilience."
About 500 passengers spent the night at Heathrow on Saturday night, according to an airport spokeswoman.
BA's chief executive Willie Walsh said it was an extremely difficult situation but they expected more robust contingency plans from BAA.
He said: "We need more resources at the airport, we need more people processing passengers at check-in and through the security systems.
"We have hundreds of volunteers from across the BA network providing assistance to our customers and I think it's important that everybody operating at the airport operates together to try and make the operation as robust as possible."
Passengers are being advised to check with airlines before setting out and be aware of new hand luggage restrictions.
Easyjet said it intended to operate as close as possible to its full programme on Sunday. However the airline urged passengers to pack all items into one piece of luggage to minimise check-in delays and possible cancellations.
Ryanair, meanwhile, called on the government to provide police or Army reserves to help carry out searches.
But the company said the measures should only be targeted towards the flights at risk or groups that posed a threat.
"The goal of these terrorists and extremists is not just to kill but also to disrupt the economic life of Britain," Ryanair's chief executive Michael O'Leary said.
"We believe that the body search requirement can and should be revised from 100% to the normal 25% of passengers without in any way diminishing airport security."
A Department for Transport spokesman said discussions between the government and BAA on dealing with the situation were continuing.
He said: "The security measures are making travel more difficult, particularly at a busy time of year, but they are necessary and will continue to keep flights fully secure."
All BA services resumed at Manchester and Birmingham airports on Saturday.