Heathrow Airport will cancel 172 flights on Monday and Tuesday after thousands of workers overwhelmingly rejected a revised pay offer.
Unite said 88% of its 4,000 union members – who include security guards, firefighters and engineers – voted to refuse the deal, meaning the parties will head to last-ditch pay dispute talks in an effort to avert a strike.
A two-day strike is set to start on Monday with further industrial action planned for 48 hours from 23 August.
Unite warned the airport against paying millions of pounds in compensation to airlines for cancelled flights instead of using the money to settle the dispute.
Unite regional officer Wayne King said: “Heathrow faces a compensation bill in the region of £4.6m from airlines if the planned strikes go ahead.
“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.”
A Heathrow spokesman said: “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 5 and 6 August, 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 re-book 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.”