Heathrow has cancelled 172 flights scheduled to depart on Monday 5th and Tuesday 6th August, with industrial action expected to create travel chaos.
Around 4,000 Unite members including engineers, firefighters and security staff voted on the airport's revised deal, with 88% opting to strike.
Heathrow said passengers should contact their airlines in the first instance to see how they may be affected.
In a statement on their website Heathrow confirmed:
We’d like to reassure you that the airport will continue to operate on these days as we put contingency plans in place. In the meantime, Heathrow remains committed to finding a solution with Unite to avoid further strike action.
Whilst most flights will operate as scheduled, to ensure your comfort, safety and security during this time, we are working with our airline partners to consolidate and reduce the number of flights operating during the strike period.
The planned industrial action also includes a further 48-hour stoppage due from 23 August.
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 August 5 and 6, and we regret that passengers looking to get away on well-earned breaks will be impacted by this.”
Passengers who are due to fly on the affected days should contact their airlines and also monitor the Heathrow website for further information, details and advice: FAQ's about industrial action at Heathrow