Heathrow Airport strike sees 172 flights cancelled and could affect 88,000 passengers.
Heathrow has “proactively” cancelled 172 flights in a move that could disrupt holiday plans for thousands of travellers.
Brits travelling this Monday and Tuesday have been told to check with their airline to see if their journey is disrupted.
Passengers can either be re-booked onto alternative services or given a refund.
British Airways – one of 91 airlines that fly out of Heathrow – could be among the most badly hit by the travel chaos.
A two-day strike is planned to start on Monday after thousands of Heathrow Airport workers voted overwhelmingly to reject a pay offer.
Unite said its 4,000 members, including security guards, firefighters and engineers will begin their strike at 00:01 on Monday, August 5, finishing at 23:59 Tuesday, August 6.
Around 88,000 passengers will be affected if the industrial action goes ahead, the union said.
Unite said it would immediately enter talks at the conciliation service Acas and warned Heathrow against choosing to pay millions of pounds in compensation to airlines for cancelled flights rather than using that money to settle the dispute.
A Heathrow spokesman said earlier: “We are disappointed that Unite has rejected the latest pay offer and will continue to seek an agreement at Acas.
“Unite is proceeding with its unnecessary strike action on August 5 and 6, and we regret that passengers looking to get away on well-earned breaks will be impacted by this.
“We have activated contingency plans which will keep the airport open and safe on both strike days. We expect security queue times to be slightly longer than normal and advise passengers to check our website for detailed information on how to prepare for their flights and when to arrive at the airport.
“As part of our plans, we are working with airlines to proactively consolidate flights and rebook passengers onto alternative services in advance.
“We also advise passengers to contact their airlines for the latest information, as well as follow our Twitter and Facebook accounts for further updates.”
The airport said it will operate throughout any industrial action in accordance with strict Department for Transport and Civil Aviation Authority regulations.
Unite regional coordinating officer Wayne King said: “Rather than provoking the disruption that strike action will cause, we would urge Heathrow Airport to use this money for an improved pay offer that better reflects the hard work of the workers who keep the airport running safely and smoothly.
“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.