Pilots at holiday specialist Thomas Cook went on strike on Friday (8 February) in a pay dispute, sparking the first strike by UK-based pilots since 1974.
The British Airline Pilots' Association (Balpa) said its member pilots walked out "extremely reluctantly" at 3am BST over deteriorating pay and working conditions.
Balpa general secretary Brian Strutton said: "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'.
Strutton added that Thomas Cook had offered a 1.5% pay rise, which was changed to 4% over two years during talks at the conciliation service.
However, the union is seeking a one-year deal "substantially" in excess of the Retail Prices Index (RPI) inflation rate, which stood at 3.6% in July, according to the Office for National Statistics.
Thomas Cook had earlier failed in its attempt to obtain an injunction preventing industrial action.
"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," Strutton added.
Thomas Cook pilots, who were not Balpa members, had ensured the strike did not have an impact on flights or inconvenience customers, the company said in a statement.
Peter Fankhauser, chief executive officer of Thomas Cook, said any pay rise had to be "reasonable and affordable" because the company was competing with budget carriers such as Ryanair and EasyJet.
He said Balpa had asked for an annual salary increase of more than 10%. "Obviously we work in an extremely tough environment. We have offered basically 4% over 2 years on top of normal annual pay increases of 1.8%. We have an offer which is clearly above inflation.
"We have moved three times. Balpa has not moved at all, so we really ask the Union to come back to the table to negotiate with us."
Escalation of the dispute sent the company's shares tumbling intraday. At 11:56am BST, Thomas Cook was down 2.61% or 3.23p at 120.67p.