Workers at Heathrow airport, including firefighters and security staff, will stage the first in a series of strikes on Tuesday in a dispute over pay and conditions.
Members of Unite will walk out for 24 hours, and again on December 14, 17 and 18 unless the row is resolved.
Unite said it has “serious concerns” about the safe operation of Heathrow during the industrial action, but the airport said it has contingency plans in place.
The strike action will involve firefighters, engineers, campus security, baggage operations, central terminal operations, landside and airside workers.
Unite officer Wayne King said: “Unite has severe concerns that Heathrow airport (HAL) will not be able to operate safely during the forthcoming strikes.
“Unite is urging the Civil Aviation Authority and the Health and Safety Executive to ensure that safety levels have not been compromised as a result of HAL’s contingency planning and that they make sure Unite is involved in those conversations.
“There are thousands of passengers and workers who will be using the airport during the strike and they require cast iron guarantees that the airport will operate in an entirely safe manner.
“Heathrow workers take great pride in their work and have continued to operate throughout the pandemic, and have been required to put their own health at risk attending to passengers who have developed Covid.
“They are taking strike action as a last resort as cuts to their pay of up to a quarter are forcing them into poverty.
“Unite has offered a variety of solutions to reduce costs and avoid industrial action, all of which have been summarily dismissed by HAL.”
A Heathrow spokesman said: “It’s very disappointing that Unite has decided to take strike action during the worst crisis to hit the aviation sector.
“Our focus has been to protect jobs, which is why we’ve offered every frontline colleague who wants one a job at a market rate salary above the London Living Wage, even despite a loss of over £1.5bn to our business so far this year.
“We do not believe this strike action is necessary, but have activated extensive contingency plans which will keep the airport open and operating safely throughout this period.”