Heathrow Airport has said that some passengers may face security delays over the Easter holidays due to strike action over pay.
Hundreds of security officers in the Unite union, who work for Heathrow Airport, have begun 10 days of industrial action.
It threatens disruption at the UK’s largest airport at the start of the Easter school holidays.
However, Heathrow said the airport was operating “as normal” on Friday.
The strike involves security guards at Terminal 5, which is only used by British Airways, and those who check cargo. Unite has accused the airport of a real-terms wage cut.
Heathrow said it had offered a 10% pay increase back-dated to 1 January, plus a lump sum payment of more than £1,000.
The airport said contingency plans were keeping the airport operating as usual.
However, British Airways cancelled about 70 flights on Friday. This included flights already removed from the schedule due to the strikes, and cancellations for other reasons, such as bad weather and an air traffic control strike in France.
Picket lines were mounted outside the airport and Unite said the strike was being “well supported”.
Heathrow chief executive John Holland-Kaye told the BBC “many” security staff had chosen to work on Friday, but “a lot” of agency security staff had been brought in, alongside “hundreds” of managers who were “here to help”.
“The airport is operating as normal,” he said.
The next few days are expected to be busy as people get away on Easter holidays.
Ahead of the strike, Heathrow asked airlines to stop selling tickets and allow customers to change travel dates.
British Airways pre-emptively cancelled 300 flights and Virgin Atlantic confirmed it had limited new ticket sales and introduced a flexible policy.
The strike reduces the number of security staff available to the airport on what is normally a very busy weekend.
Heathrow said it was deploying 1,000 extra colleagues and its management team to assist passengers.
Travellers should check their flight before travelling to the airport, arriving at Heathrow no earlier than two hours before short-haul flights and three hours before long-haul flights, and be ready for security, the airport said.
Passengers will only be permitted to go through security with two items of hand luggage to help the flow.
The aviation industry more broadly is under pressure from the government and the industry regulator to avoid a repeat of last year’s Easter’s queues, delays and cancellations, which were largely caused by staff shortages.
A leaked letter to businesses from the Civil Aviation Authority and the Department for Transport, seen by the BBC, says it would be unacceptable for consumers to face the same level of disruption this year.
Airlines and airports have told the BBC they are confident of having enough staff in place this time round, although external factors such as strikes in France affecting air traffic control could cause issues.