Heathrow Airport has cancelled 177 flights on Monday and Tuesday after a union vote rejected a pay offer.
Around 4,000 Unite members including engineers, firefighters and security staff voted on the airport's revised deal, with 88% opting to strike.
Unite says almost 2,500 staff will miss work if the walk-outs go ahead.
Heathrow has not said which flights will be cancelled, and said passengers should check with their airlines to see if they are affected.
Affected passengers might be offered other flights or refunds by their airline, it added.
The cancellations affect 91 airlines including British Airways.
The airport said on Twitter it had advised each airline on the number of flights it needed to cancel - but added it was the airlines' decision to choose which ones to cut.
British Airways said on Twitter it had no information which flights it plans to cancel.
Talks between union leaders and Heathrow management at the conciliation service Acas, aimed at averting the strike action, were held on Friday and Saturday and are expected to continue on Sunday morning.
After the talks were adjourned, a Heathrow spokesperson said: "Unite have rejected our revised pay offer today.
"While we are disappointed, we will continue to seek an agreement at ACAS tomorrow [Sunday]."
The airport, which advised passengers to check its website for updates, said its contingency plans would keep Heathrow open and safe on both strike days, albeit with some disruption.
Passengers still scheduled to fly on Monday and Tuesday have been warned to arrive at least three hours before long-haul departures and two hours ahead of short-haul departures, because it may take longer to get through security.
A Heathrow spokeswoman said earlier: "I can confirm that we are working with our airline partners to consolidate and reduce the number of flights operating during the strike period.
"We have proactively cancelled 177 flights departing Heathrow across Monday and Tuesday.
"Passengers on these flights will have either been rebooked onto alternative services or provided a refund."
Unite regional co-ordinating officer Wayne King said: "This latest vote for strike action points to growing anger among the airport's workers in a whole range of vital jobs which are essential to the smooth and safe running of Heathrow.
"Airport bosses need to heed this latest strike vote and the overwhelming rejection by our members of the revised pay offer which offers little over and above the original offer of £3.75 extra a day for many workers."
Meanwhile, talks aimed at averting a separate strike by British Airways pilots are to continue next week.
Leaders of the British Airline Pilots Association (Balpa) met the company last week to try to resolve the dispute over pay.
The union would have to give two weeks' notice of any industrial action.
Can I claim compensation if my flight has been cancelled?
If your flight out of Heathrow has been cancelled, you should contact your airline to see what you are entitled to in terms of a refund or compensation.
However, if your flight has been cancelled due to airport (rather than airline) staff striking, it is unlikely you will be able to claim compensation as this would be considered "extraordinary circumstances" outside of the airline's control, the Civil Aviation Authority said.
The CAA added that the airport is not obliged to pay compensation directly to passengers, and whether the airport gives its customers (the airlines) compensation is a commercial issue between the two parties.
If your flight has been cancelled because airline staff are striking, the CAA said, then this would be considered within the airline's control, and therefore you have a legal right to either:
- A full refund, and this includes flights in the same journey that might be from a different airline (for example, an onward or return flight)
- A replacement flight to get to your destination
- Or, if you are part way through your journey and don't want a replacement flight, you are entitled to a flight back to the airport you originally departed from
If the cancellation delays you by two hours or more, you are also legally entitled to compensation and help with any costs you may incur as a result of the delay.
Are you due to fly on Monday or Tuesday? Has your flight been affected? Get in touch by emailing firstname.lastname@example.org
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