A new strike by ground handlers is planned over the new year after a revised pay offer was rejected.
A walk out planned for tomorrow (Friday) will now go ahead for 72 hours.
A further strike by more than 400 workers employed by Menzies is also to be staged between December 29 and the early hours of January 1.
The post-Christmas stoppage will also coincide with the planned walkout by Border Force staff.
The dispute involves workers operating from Heathrow terminals 2, 3 and 4.
The Unite union members rejected a 4% pay rise backdated to May and then a further 6.5% pay increase from January 1. They are seeking an increase of 13% for 2022 alone.
Unite regional officer Kevin Hall said: “Heathrow passengers need to brace themselves for fresh disruption at the airport purely because of Menzies intransigence.
“Strike action is being taken as a last resort because Menzies has only been prepared to offer our members a miserable pay increase that in no way reflects the cost of living.”
Menzies Aviation Europe executive vice president Miguel Gomez Sjunnesson said: “We are extremely disappointed that Unite has rejected the pay increase despite GMB accepting the offer.
“Our offer of a 10.5% salary increase is extremely competitive and reflects feedback from our employees.
“It is a great shame that as a result of Unite rejecting the offer, strike action will now fall on one of the busiest weekends of the year as people travel to meet family and friends for Christmas.
“We are well prepared for industrial action and have been working closely with key partners to put in place robust contingency plans.
“We remain committed to ensuring that our employees receive their increase as soon as possible and hope to be able to resolve this matter soon.”