Thomas Cook pilots are staking a 12-hour strike today in a dispute over pay.
The walkout from 3am by members of the British Airline Pilots' Association (Balpa) is believed to be the first strike by UK pilots since the mid 1970s.
Union members voted by 9-1 to take industrial action.
Thomas Cook failed on Wednesday to obtain an injunction stopping the action after the company unsuccessfully argued that the ballot was invalid.
BALPA general secretary Brian Strutton said: "BALPA has always been confident in the decisive ballot result and was disappointed that Thomas Cook decided to challenge it legally rather than spending time and effort bringing an acceptable offer to the table.
"We have taken this course of action extremely reluctantly, but with no sensible pay offer on the table, we have no other option. Thomas Cook pilots have faced year-on-year, real-terms pay cuts, and cuts to terms and conditions, and our pilots have said 'enough is enough'.
"We have, however, kept this action to a minimum - just 12 hours. Thomas Cook have stated that all of its flights will operate - we hope that this is true, as Thomas Cook's customers are not to blame for this action.
"But we also believe that if Thomas Cook had spent time negotiating rather than rearranging schedules, this action may well have been unnecessary.
"We remain committed to finding a solution to this dispute which will mean Thomas Cook putting forward a pay offer that their pilots can accept."
A Thomas Cook Airlines spokesman said: "Our priority is to make sure all of our customers can still fly on holiday and back home with us during the strike period and we are pleased to confirm that our flights on September 8 will operate."
Thomas Cook offered a 1.5 per cent pay rise, which was changed to four per cent over two years during talks.