British Airways staff have returned to work following an unofficial walkout at Heathrow Airport.
80,000 passengers were affected by the strike
An estimated 80,000 passengers have been affected by the action, which started on Friday.
The walkout resulted in inward and outbound flights to domestic and European destinations being cancelled, with all 360 scheduled flights from Terminal One grounded on Saturday.
Saturday's short-haul services from Terminal Four were also cancelled, along with all long-haul flights before 1800 BST.
BA say some travellers could face a wait of up to two days.
A spokeswoman said: "It will take some time to clear the backlog and the flights that were cancelled will remain so.
"We will be working to book the passengers back onto flights so they can finally begin their summer holidays - and after this they probably need them more than ever."
Passengers travelling on long-haul flights scheduled for after 1800 BST were "checking in normally" but "there may be some disruption and delay", the spokeswoman added.
"Talks with the unions are ongoing," she said.
A union source said staff had returned as "a gesture of goodwill to passengers".
Passengers have criticised BA's response to the industrial action, saying no staff are on hand to help those stranded - causing "human gridlock".
John Pritchard, who had been due to fly to Geneva, told BBC News Online frustrated passengers had started to throw punches and police had stepped in to calm things down.
"It is chaos, there are no BA staff around at all," he said
"All check-in desks have been closed and all electronic check-in machines have been switched off. It is human gridlock."
The BA spokeswoman said: "Passengers who are turning up for flights will be handed letters explaining what is happening and how to get a full refund.
"Customers have three options - re-book, re-route or refund."
The unofficial strike by 250 ticket and baggage handling staff began at 1600 BST on Friday, forcing BA to cancel flights from Terminal One. Staff also stopped work in Terminal Four.
Hundreds of passengers, unable to find hotel accommodation, spent Friday night in the terminal.
About 2,000 passengers hoping to travel between Scotland and London were stranded overnight.
Many passengers spent the night at the terminal
The workers were protesting against a new swipe card entry system, called Automated Time Recording, which allows managers to monitor their working hours.
It is understood staff are worried that the system, due to be introduced on Tuesday, could lead to staff being sent home during quiet periods.
BA denied this, and said swipe cards had already been in use in some parts of its Heathrow operations for three years and was widespread across British industry.
But one union official said managers had been warned staff were deeply
unhappy about the system.
He said: "These are not militant workers - but they have just had enough."
The Transport and General Workers' Union (TGWU) - which represents some BA staff - confirmed that the strike was unofficial, as the move had not been put to the vote by its members.
BA has set up a special telephone line for passenger information, on 0800 727800.