Talks to resolve a pay issue with Heathrow staff have ended without an agreement.
The airport plans to cancel 177 flights on Monday and Tuesday after union members voted overwhelmingly to reject a pay offer.
The Unite Union said 88 per cent of its 4,000 members - which includes security guards, firefighters and engineers - voted down the deal.
Industrial action will currently take place on August 5 and 6, with further strikes planned for August 23 and 24.
Those due to fly on days of industrial action have been offered a refund or allow to rebook alternative flights.
Crunch-talks, which happened on Saturday, tried to find a consensus to avoid thousands of Heathrow workers walking out.
Passengers scheduled to fly on Monday or Tuesday have been told to arrive at least three hours ahead of long-haul departures, and two-hour ahead of short-haul departures, as it may take longer getting through security checks.
People are being advised to double check their flight status with airlines before leaving for the airport.
What has Heathrow and Unite said about the strike action?
Unite's members voted by almost 9-1 to reject a pay offer Heathrow said was worth 7.3% over two and a half years.
The union has told the airport not to pay millions of pounds in compensation to airlines for cancelled flights, but instead use the money to settle the dispute.
Wayne King, Unite regional officer, said: "Heathrow faces a compensation bill in the region of #4.6 million 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.
A Heathrow spokesman said earlier: "We are disappointed that Unite has rejected the latest pay offer and will continue to seek an agreement at the Advisory, Conciliation and Arbitration Service (Acas)."
They added: "We also advise passengers to contact their airlines for the latest information, as well as follow our Twitter and Facebook accounts for further updates."
A row flared before any strikes started, when the firefighters' union accused Heathrow of planning to hire a "privatised strike-breaking outfit" from Surrey County Council to replace crews taking industrial action.
The Fire Brigades Union said South East Business Services was set up by the council over four years ago as a separate local authority trading company.
Matt Wrack, FBU general secretary, said: "This is a backhanded act of strike-breaking from Surrey County Council."
What should you do if you're flying?
The UK Civil Aviation Authority, which oversees and regulates all aspects of air travel, told ITV News to check before you travel.
In a statement, they said: “During the proposed industrial action at Heathrow Airport, passengers have the right to be looked after by their airline, in line with European regulations. "This includes being provided with meals and refreshments if a flight is delayed by two hours or more, and hotel accommodation if the delay is overnight.
“Passengers affected by the strike action, however, are not automatically entitled to flight delay compensation. Much will depend on the circumstances of the strike and whether the individual airline took all reasonable measures to minimise disruption for their passengers.
“Industrial action carried out by airport employees or contractors, such as air traffic controllers and airport security staff are considered to be outside the airline’s control, and therefore an ‘extraordinary circumstance’ for which airlines are not obliged to pay compensation.”
If you are travelling from Heathrow on the day of the proposed strikes, here are some things to bear in mind:
If a passenger’s flight is cancelled their airline must rebook them on to an alternative flight, or offer a full refund for the entire booking (ie. return flight as well).
If a passenger experiences a delay at the airport while waiting for their flight, their airline must provide care and assistance (meal vouchers etc) regardless of the cause of the delay.
If a flight cancellation or delay is the result of industrial action taken by third-party employees (ie. airport staff) then an airline is probably not obliged to pay compensation to passengers.
Airports are not legally obliged to pay flight delay compensation to passengers.