More than 1,000 workers are currently voting on whether or not to stage a walkout.

The dispute centres around alterations to rest breaks at the airline. Cabin crew are now only allowed one 20-minute rest break for every 11 hours, 59 minutes worked. Before the change, they were allowed a 20-minute break ever six hours.

If union members vote for a strike it could affect the upcoming half-term school holidays.

Unite national officer Oliver Richardson said: “Our members are deeply concerned about the impact these changes to rest breaks are having on inflight safety.

“Fatigue is a major contributory factor to accidents and slower response times when there is an incident.

“The primary purpose of cabin crew is the safety of passengers, but instead Thomas Cook seems intent on working them to the bone to extract as much money as possible out of passengers at the expense of safety."

A Thomas Cook spokesperson said: “We would like to reassure our customers that nothing matters more to us than safety. It’s regrettable the union has chosen this path because the crew rest procedure, which includes monitoring all crew rest on all flights, was introduced with the agreement of the union.

“It also meets the regulations of industry experts the Civil Aviation Authority and does not compromise on safety.

“We have offered to meet union representatives and the message we hear directly from our crew is that they’re looking forward to a great summer of flying customers on holiday.”

Unite said cabin crew will typically work 10 to 15 hour shifts and up to 60 hours a week.

The ballot of cabin crew closes on Wednesday May 25.