Aircraft could be delayed, disrupted and grounded at Heathrow over the May bank holiday weekend due to a strike by refuelling staff.
The warning was made by the Unite union as it said 50 workers are to take industrial action for 72 hours from May 4.
The threatened walkout is due to a dispute with employer AFS over cuts to the terms and conditions of new staff recruited since January.
Unite members at AFS refuel aircraft at the London hub for 35 airlines including Air Canada, Air France, American Airlines, Delta Air Lines, Emirates, United Airlines and Virgin Atlantic.
“Strikes over the May Day bank holiday are likely to lead to severe disruption to airline services for passengers,” Unite warned.
AFS is a joint venture between oil and gas companies Air BP, Total, Q8 Air and Valero.
Unite regional officer Kevin Hall said: “Our members recognise that if they allow this attack on their conditions to take place, before long this will become the norm. Unite won’t allow that to happen and is standing firm with our members on the picket line.
“AFS will have to answer directly to the airlines and passengers for the disruption caused by their unmerited actions.”