Heathrow said the airport was operating as usual on Friday despite a strike by security guards in a dispute over pay.
Around 1,400 members of Unite will be on strike for 10 days, covering much of the busy Easter weekend, after last-ditch talks broke down on Thursday evening.
Picket lines were mounted outside the airport and Unite said the strike was being “well supported.”
The airport said its contingency plans were working well, although some British Airways flights are expected to be cancelled.
By mid-morning a total of 42 flights to and from Heathrow on Friday had been cancelled, according to the airport’s website.
The airport’s chief executive John Holland-Kaye told Sky News: “I’m here in Terminal Five which is the only terminal that voted for strike action and you’ll see it’s operating as normal.
“We have a lot of colleagues who have come to help us out today – both security officers and managers who are helping out in their purple shirts like me.
“But we also have some other agency, third party, workers who have come in to help us who are very experienced in this kind of security environment, and they’re keeping the airport running smoothly.
“So, Heathrow is operating as normal. If you’re travelling over the Easter period, don’t worry, you’ll have a good journey.
“Please don’t come too early. Three hours is plenty for a long-haul flight, two hours for a short-haul flight, and we will get you to your destination on time.”
Unite general secretary Sharon Graham said: “Heathrow can afford to pay a decent pay rise to its workers.
“This is a wealthy company which is about to return to bumper profits. In recent years it’s approved an astronomical rise in salary for its chief executive and paid out dividends to shareholders worth billions.
“Yet somehow, Heathrow executives seem to think it’s acceptable to offer what amounts to a real-terms pay cut to its security guards and ground staff who are already on poverty pay.
“Unite has a laser-like focus on our members’ jobs, pay and conditions. The workforce at Heathrow Airport will receive the union’s unswerving support in this fight for a decent deal.”
“The strike action will undoubtedly result in severe delays and disruption to passengers across the airport but this dispute is a direct result of Heathrow Airport’s stubborn refusal to pay its workers fairly.”
Picket lines will be in place across the airport throughout the 10 days of continuous strike action, which ends at 23.59pm on Easter Sunday April 10.
The strikes involves security officers at Terminal Five, which is used exclusively by British Airways, and campus security guards who are responsible for checking all cargo that enters the airport.
BA said in a statement: “Due to a number of issues including the Heathrow Airport strike, continued French Air Traffic Control industrial action and bad weather, we’ve been forced to make a small number of adjustments to our schedule.
“We’re sorry for the disruption to our customers’ travel plans.
“The vast majority of our flights continue to operate as planned and we’re in contact with affected customers to inform them of their rights and offer them options including a full refund or rebooking on to an alternative flight.”
“It’s essential Heathrow is prepared for the passengers whose flights will go ahead, and that no traveller misses their flight due to long lines at security and is left out of pocket.
“Anyone yet to book their holiday may want to consider alternative airports.
“French air traffic control strikes are also threatening wider chaos for travellers.
“Airlines should ensure they keep passengers informed of any potential changes to their booking as result of strike action as early as possible, and must not fail in their legal responsibility to offer travellers a refund or the option to be rebooked if their flight is cancelled, including with other airlines if necessary.”