Ground staff at Heathrow Airport have begun a 72-hour strike, in protest at their pay.
About 350 workers started the action from 4am on Friday, according to the Unite union.
The strike by workers at aviation services firm Menzies will affect a range of airlines and disrupt a number of flights leaving terminals 2, 3 and 4 at Britain's busiest airport, a statement from the union said.
Flights operated by Air Canada, American Airlines, Lufthansa, Swiss Air, Air Portugal, Austrian Airlines, Qantas, Egypt Air, Aer Lingus and Finnair are most likely to be disrupted.
Workers are walking out in protest at what the union describes as a "derisory" pay offer from Menzies to the ground staff, which they say is "far below the current real inflation rate of 14.2 per cent (RPI) and amounts to a substantial real terms pay cut".
It had been feared passengers using Qatar Airways to fly to the football World Cup might be affected, but planned action by another ground handling company, Dnata, and by Menzies Cargo, has been cancelled.
Alex Doisneau, managing director, of Dnata UK, Airport Operations (Ground Handling & Cargo), said: "We are pleased to have reached agreement on this pay award, which reflects the huge respect we have for our employees in the UK.
"The agreed package, including both a pay increase and immediate support for the short term, provides concrete support for our employees amidst the current cost-of-living crisis."
Unite regional officer Kevin Hall said Friday's 72-hour walkout was "entirely of Menzies' own making".
He added: "They have had every opportunity to make a fair pay offer, but have chosen not to do so."
A Heathrow spokesperson said it was aware of the proposed industrial action, and it was "in discussions with our airline partners on what contingency plans they can implement to support their ground handling".
"Our priority is to ensure passengers are not disrupted by airline ground handler shortages," they added.