Rail union RMT will begin balloting Heathrow Express staff today for strike action in response to cuts in pay, jobs and safety.
The ballot for strike action and action short of strike will close on April 15.
The union claims Heathrow Express is planning to cut nearly half of its workforce as it attempts to save £6 million over the next five years.
Heathrow Express, which charges £21 for a 15-minute journey, is cutting costs in anticipation that it will be affected by competition from Crossrail services.
From 2019 Crossrail will allow passengers to reach Heathrow direct from the City and Canary Wharf without having to change at Paddington.
RMT acting general secretary Mick Cash said: "The Heathrow Express and Heathrow Connect services are dealing with growing demand as the airport expands and the idea that safety and service is going to be sacrificed at the stroke of an accountant’s pen would shame London and leave these high-profile services extremely vulnerable."
Keith Greenfield, managing director for Heathrow Express, said: “Our proposals offer all existing employees a future role within Heathrow Express.
"Crossrail will be the first direct competition to us since our launch in 1998. Our proposals will cut our operating costs by nearly £6 million over the next five years, while avoiding the need for compulsory redundancies and protecting existing employees’ futures with the company."