Stringent security searches which have led to long delays and cancellations at Heathrow are not sustainable, airport operator BAA has warned.
More queuing is inevitable if extra checks continue, Heathrow says
The airport cancelled a third of flights on Saturday evening in a bid to speed its return to a normal schedule.
Heathrow earlier came under fire from British Airways for being unable to cope with the extra security measures.
Meanwhile, Ryanair said the government should provide additional staff to carry out body searches at airports.
The British Airports Authority said Heathrow was suffering severe delays, meaning that a third of departing flights, between 1500 and 2330 on Saturday evening, would be cancelled.
It said the remaining flights should depart from Heathrow on schedule or as close to schedule as possible.
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
BAA said the decision was "extremely regrettable" but added that it was the only way services could return to normal.
Tony Douglas, BAA's chief executive officer for Heathrow, earlier spoke about how the extra searches were affecting the airport.
He said: "If this is maintained we are likely to continue to see extremely long queues and regrettably even more flights cancelled."
He added: "Quite simply I don't know how long it's likely to go on, but it's clearly a set of measures that are unprecedented and by virtue of what they've come in to enforce, they're not sustainable measures."
But Willie Walsh, British Airways chief executive, criticised Heathrow and insisted his company was ready to fly a full schedule.
He said: "The airport's baggage system cannot process all of the passengers' bags and, where passengers have been able to check their bags in, the lengthy queues in the airport security search area means that passengers are unable to get to the departure gate in time for their flight."
He added that BA had to cancel 25% of its short-haul services on Saturday while "many" other flights left without all passengers.
Mr Walsh went on: "We are ready and able to operate a full schedule at London Heathrow. We have sufficient flying crew, ground staff and aircraft in place.
"However BAA is unable to provide a robust security search process and baggage operation at London Heathrow and as a result we are being forced to cancel flights and operate some others from Heathrow without all the passengers onboard."
Ryanair's chief executive officer Michael O'Leary said BAA needed help from the government.
He said: "If the British government is serious about defeating terrorism and not allowing the terrorists to disrupt normal everyday British life, then the government must provide the additional security staffing - either police or army reserve personnel - immediately to prevent London's main airports from grinding to a halt over the coming days."
Virgin's spokesman, Paul Charles, echoed sentiments over extra resources.
He said: "In some cases, Heathrow is definitely under a lot of pressure. Our check-in area actually is very good this afternoon and we're processing passengers very well.
"BAA is doing the best job it can in what is a very difficult environment, but clearly extra resources would be useful."
A spokesman for the Department for Transport said: "We understand the situation remains difficult. Government is providing assistance on many levels and we are working intensively with the airlines and airport authorities.
"In particular we are discussing with BAA, the operators of Heathrow, Gatwick and Stansted, on how the Government can continue to work with them in dealing with the situation."
British Airways has cancelled 43 outbound and inbound flights, which amounts to 25% of their shorthaul service.
A spokesman said: "We have had 86 flights affected by the continuing disruption.
"We would not anticipate it being any better tomorrow unless something changes in the way BAA operates at Heathrow or does something to reduce the tremendous congestion at the airport."
He said the situation was being handled more efficiently at Gatwick where there were no further cancellations expected in addition to the 10 domestic flights that have already been announced.
All BA services had now resumed at Manchester and Birmingham airports subject to delays.