Thousands of air travellers are facing ongoing delays and flight cancellations because of extra security searches and a ban on most hand luggage on planes.
Tony Douglas, Heathrow Airport's chief executive officer, said the measures were "not sustainable" long term.
He said: "If this is maintained we are likely to continue to see extremely long queues and regrettably even more flights cancelled."
Air operator BAA advised passengers to check schedules before setting off.
Heathrow - which has been worst affected - said it was still expecting some delays and cancellations.
Hundreds of passengers are still having to queue outside the airport's terminals under specially erected marquees.
British Airways has cancelled 10 round-trip long-haul flights and 31 round trip short-haul flights on Saturday from that airport.
CABIN LUGGAGE ALLOWED
Heathrow has issued a statement which said: "The airport is slowly returning to normal but is still severely disrupted.
"We are advising all passengers to call their airlines before travelling."
Virgin Atlantic said it was opening check-in desks four hours ahead of departure times to give passengers time to clear security checks.
EasyJet is opening its desk three hours ahead of flying times.
In and out of Gatwick, British Airways cancelled 10 round-trip domestic flights for Saturday.
All flights to European destinations were planned to operate as normal, with no long-haul cancellations expected.
Stansted Airport said 31 flights had been cancelled, although after an initial flurry the airport was "a lot quieter" with passengers being processed smoothly.
All airports warned passengers to check their flights before leaving home and to arrive early and without hand luggage.
Simon Calder, travel editor of The Independent, told BBC News 24 the situation for passengers at Britain's airports had improved although those heading to some sunshine destinations were worst affected.
"There's only been a couple of cancellations and many flights have got away, albeit with short delays.
"All cabin baggage must be processed as hold baggage and carried in the hold of passenger aircraft departing UK airports"
"EasyJet says it plans to run a completely full programme. BA's cancelled from Gatwick a number of domestic flights, as it did [on Friday]. And Ryanair says that one in five of its flights to and from Stansted won't be operating.
"Crucially those, unfortunately, are many holiday flights to places like Spain, France and Italy." Abta spokeswoman Dee Byrne said the backlog caused by cancellations and delays meant disruptions "will probably continue...into the start of next week".
However, she stressed that effects varied according to different airlines.
She went on: "People should continue to travel and take it that their holidays and flights are going to happen, unless they are told otherwise - but allow extra time to check in."
By way of example, she pointed out that "tour operators managed to get most people out on their holidays, but suffering some delays."
On Friday afternoon, ministers reiterated to airline and airport operators that current hand-baggage restrictions were only expected to be in place for "a limited time".
The reassurances came at a meeting of the National Aviation Security Committee, chaired by the Transport Secretary Douglas Alexander.
A Department for Transport spokesman said any alterations to the current security level would be conveyed to operators and airports "in due course".
Under the new guidelines, travellers must 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.
Apart from passengers travelling to the US, travellers will be allowed to take purchases made in departures lounges onto planes.
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