Some 35 airlines, including the likes of Virgin Atlantic, Delta Airlines, Emirates and Air France, are set to be affected by a strike by refuelling operatives at Heathrow airport, starting Saturday 4 May.
The union said the 72-hour walkout risked aircraft being delayed, disrupted or grounded.
Around 50 refuellers working for AFS, a joint venture between several fuel firms including Air BP and Total) will join the action next month in a dispute over changes to terms and conditions for new staff recruited since January.
According to Unite, these include reduced pension and sickness benefits.
Sharon Graham, Unite general secretary, described AFS’s behaviour as "appalling bully-boy tactics".
Kevin Hall, Unite regional officer, added: “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."
Other airlines that could be affected include Air Canada, American, United, Qantas, JetBlue, Tap Air Portugal, Latam and Air Mauritius.
