Thomas Cook Group PLC (LON:TCG) shares flew lower after its pilots said they were planning three more strikes after a 12-hour stoppage today in a dispute over pay.
The airline said all flights had operated without disruption after members of the British Airline Pilots' Association trade union (Balpa) walked out at 3am today.
It marked the first strike by UK pilots since 1974, according to the union.
Balpa said its members will strike again on 23 and 29 September, and 3 October, if they are unable to agree a new deal on pay.
Thomas Cook said it offered pilots a 4% increase in pay over two years on top of automatic annual increases, including 1.75% in first year and 2.25% in the second.
But the union wants a one-year deal “substantially” in excess of the retail price index inflation rate, which came in at 3.6% in July, along with other aspects of remuneration for pilots.
Thomas Cook's chief airlines officer, Christoph Debus, said Balpa’s demands added up to more than a 10% annual pay rise, or around £10,000 per pilot.
Pay rises must be reasonable amid competitive ennvironment, says Thomas Cook
Debus aid any increase in pay had to be "reasonable and affordable" as the company has faced fierce competition with low cost airlines including Ryanair and EasyJet.
He said the airline has also been hit by rising costs associated with a weaker pound since it has had to pay for fuel in US dollars and other aircraft expenses in euros.
"Obviously we work in an extremely tough environment,” Debus said.
"We have an offer which is clearly above inflation. We have moved three times.
"Balpa has not moved at all, so we really ask Balpa to come back to the table to negotiate with us."
Thomas Cook pilots' wages falling in real terms, argues union
Balpa general secretary Brian Strutton said Thomas Cook pilots have had real-term annual pay cuts and worse terms and conditions.
Strutton said the pilots had “no other option” than to strike.
He welcomed the news that Thomas Cook had ensured the strike did not have an impact on flights or had inconvenienced customers.
The carrier said it was able to reschedule its flights and bring in pilots, who were not members of the union, to fly its aircraft.
“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,” said Strutton.
Shares fell 3.07% to 120.10p in afternoon trading.