Workers who operate luggage scanning equipment at Stansted Airport have begun a 24-hour strike because of a deadlocked row over pay, union leaders said.
More than 30 members of the GMB are taking industrial action after rejecting a 1.5% pay offer from their employers Airfield Services.
The union is mounting a picket line at the airport in Essex, which will be used by 40,000 departing passengers during the strike.
The GMB accused Airfield Services of refusing to go to the conciliation service Acas to settle the dispute which the union estimated would cost a total of £300 a week to resolve.
General Secretary Paul Kenny said it was "grossly irresponsible" of the company to refuse an offer of conciliation, adding: "The company's claim that they cannot afford to settle the dispute is not credible since they are owned by 40 major financial institution worth billions of pounds."
Stansted said it expected minimal or no disruption to flights because of the contingency plans put in place to deal with the walkout.
Bar staff at the Reading and Leeds music festivals will donate money to the strikers.