Heathrow Airport is to cancel 172 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.
If the walk-outs go ahead as planned on 5 and 6 August, Unite said close to 2,500 staff will miss their shifts.
Heathrow said passengers should contact their airlines in the first instance to see how they may be affected.
Affected passengers may be offered other flights or refunds by their airline, it added.
The airport said its contingency plans will keep Heathrow open and safe on both strike days, albeit with some disruption.
Heathrow advised passengers to check its website for updates.
Now, Jeff will ask why off topic, why not UK...
Because our worldwide membership may have family trying to fly home