Almost 450 Heathrow workers providing assistance to passengers with restricted mobility (PRM) could go on strike in a row over pay.
The members of the Unite union are being balloted for strike action.
Any walkout would impact PRM services across the entire airport, the union warned.
The ballot for strike action closes on March 20.
Unite claimed the workers employed by specialist contractor Wilson James are angry at being paid an average of 9.5% less than equivalent staff at Gatwick performing the same role.
Unite general secretary Sharon Graham said: “There is absolutely no justification for Wilson James paying its Heathrow workers significantly less than its Gatwick staff doing the same job.
“Both Wilson James and Heathrow are extremely wealthy. The money is there to close the pay gap – Unite will support our members 100% to ensure that happens.”
Unite regional officer Martin West added: “There is still time to avoid industrial action but Wilson James must come back with an acceptable pay offer.
“Heathrow is thriving and we urge the airport’s management to step in and ensure that its Wilson James workers are properly compensated for their hard work and dedication.”
A Wilson James spokesperson said: “We are disheartened that we are yet to reach an agreement for the review of pay to our colleagues at Heathrow airport.
“The company has put forward an above inflation pay offer to our team, which would secure a 5.16% pay increase. This increase would take hourly pay to at least 21% above National Living Wage.
“We are aware that Unite did not ballot their members on our pay offer so members have not yet had the opportunity to accept or reject this.
“Wilson James remains committed to seeking a resolution through continued discussions with Unite and has contingency plans in place to ensure that the assistance service will always be available to support passengers at Heathrow.”