More British Airways flights have been cancelled as a second strike by the firm's cabin crew gets under way.
According to live departure boards, 93 scheduled flights which were due to depart from Heathrow on Saturday have already been cancelled.
BA has said it expects to fly 75% of passengers and predicts less disruption than during last weekend's strike, when cabin crew walked out for three days.
The strike is due to run from Saturday until Tuesday 30 March.
Steve Turner, from the Unite union, warned further strike action was likely to occur after the Easter period unless an agreement was reached over changes to pay and conditions.
Contingency plan
The airline expects to fly more than three-quarters of its passengers.
BA said customers affected by the strike had been informed of flight cancellations five days ago.
Of the 240,000 customers originally booked to fly in the strike period, 180,000 will fly either on BA aircraft, or on those hired from other carriers.
Another 43,000 have rebooked with other carriers or changing flight dates, with around 17,000 left unable to travel because of strike cancellations, BA said.
London City was operating as normal, and all flights in an out of Gatwick were expected to be unaffected the strike, BA said.
It said dozens of flights had already departed from Heathrow - as part of an increased flying schedule - and at least 70% of long-haul flights and 55% of short-haul flights are expected to operate normally.
The numbers of cabin crew reporting at the airport were "currently at the levels we need to operate our published schedule", BA said.
Hundreds of strikers have been gathering at a football ground close to Heathrow since 6am and are taking turns to form several picket lines around the airport.
One woman, who wished to remain anonymous, told the BBC she had worked for BA for 11 years and loved her job, but felt she had to "look at the bigger picture" and "show her support for the Union".
She said although she could understand cuts were "to a certain degree" necessary, her basic pay was £11,000 and cabin crew "allowances" could be low. The Union had put a good agreement forward, and BA and the Union "should meet somewhere", she said.
Comparing her wage to that of BA chief executive Willie Walsh, she said cuts would be more acceptable if they were "across the board".
A total of 28 flights due to fly out of Heathrow on Saturday morning - and 65 on Saturday afternoon and evening - have been cancelled, according to departure boards on BA's website. Five morning and 32 afternoon/evening arrivals into Heathrow have also been cancelled.
At Manchester airport, one out of 12 departures and one out of 10 arrivals were cancelled on Saturday morning.
BA says it has deployed "the biggest contingency plan in our history" to try and limit the impact of the strikes.
Despite those measures it estimates that last weekend's three days of stoppages cost the company a total of £21m - about £7m a day.
But Unite national officer Brian Boyd told the BBC some City analysts had estimated the seven days of action would cost BA about £105m.
Stacked up against the £10m BA needed to resolve the dispute - it "just did not make sense", he said.
'Macho' Walsh
The latest stoppage strikes comes amid controversy over BA's conduct during the industrial dispute.
In a letter to the Guardian newspaper on Friday, a total of 116 industrial relations academics accused chief executive Mr Walsh of adopting a "macho" approach, aimed at breaking the power of the Unite union, which represents BA cabin crew.
However BA rejected the accusations, pointing to the three days of negotiations with Unite attended by Mr Walsh in the run up to the first strike.
Speaking to the BBC, Mr Walsh said that there were currently no plans to reopen talks with the union.
He also said that travel perks withdrawn from striking staff would never be reinstated. At the company's discretion, BA staff are able to buy flights for 10% of the face value - a deal that can be extended to friends and family.
Unite called the withdrawal of the perks "unacceptable anti-union bullying".
Unite's Steve Turner said the union was "absolutely crystal clear" there would be no settlement without "the staff travel concessions that are really important to our members having been returned".
He said further strike dates were likely unless a sensible proposal was put on the table, but stressed action would not affect "hard working families wanting to take holidays over the Easter period".
BA and Unite are in dispute over the airline's cost cutting plans, which include reducing the numbers of cabin crew on long-haul flights.
The union says that the plans involve contractual changes for its members, which it says it was not consulted about.
But BA says the High Court ruled the changes were reasonable in February, and the Union had been consulted for several months.
Analysts say BA needs to bring down its costs significantly. It is expected to announce the biggest loss in its privatised history when it reports its annual results later this year.
Last year it reported an annual loss of more than £400m.
Aviation strategist Laurie Price said the only people that were going to lose out were "those going on strike", with privileges, opportunities, and potentially jobs at risk.
"All it's going to do is make the management perhaps more resolute. It's going to make those passengers who are committed to fly with British Airways equally resolute.
"The security of [the staff going on strike's] full employment is going to be adversely affected because no business can afford to continue to make the sort of losses that British Airways has been".
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