Heathrow said the airport is operating as usual on Thursday despite a strike by security guards.
Members of the Unite union walked out in the first of a series of stoppages after talks over a pay dispute broke down on Wednesday evening.
Heathrow said its contingency plans are working well and no flights have been cancelled as a result of the industrial action.
A Heathrow spokesman said: “Passengers can be reassured that they will travel as normal. This is an important time for the country, and we will not let these unnecessary strikes disrupt journeys.
“We have activated our contingency plans and deployed 750 additional colleagues and the entire management team who will be on hand in the terminals providing assistance to passengers.
“The majority of colleagues do not support strike action. Colleagues could have an inflation-matching pay increase for two years – 10% this year and a CPI-linked increase in 2024, as well as a £1,150 lump sum, but instead they’re left empty-handed by Unite’s refusal to allow members to vote on the offer.”
“Sadly, they refused to grasp the opportunity to make an offer which could meet members’ expectations.”