The main pilots’ unions, Vereinigung Cockpit (VC), had already extended Wednesday’s planned single-day strike to Thursday.

So far the strike has led to the cancellation of 1,800 flights, including many long-haul departures - with services from the main Frankfurt hub to cities such as Hong Kong, Johannesburg and San Francisco grounded. Around 215,000 passengers have been affected. Late on Wednesday night, the union said it would ask pilots on domestic and European flights to stop work on Friday. While the third day will not have the same impact as the first two, tens of thousands of passengers are likely to see their flights cancelled.

The union said: “The VC regrets the impact on the passengers, cabin crew and ground staff concerned.”

Lufthansa said it “will do everything possible to keep the impact as low as possible”.

The dispute is over a pay claim. The pilots’ union Vereinigung Cockpit (VC) says that its members’ salaries are lagging behind while Lufthansa makes healthy profits. It is demanding a 20 per cent rise to cover the years from 2012 to 2017. The airline has offered 2.5 per cent.

Lufthansa is facing intense competition from low-cost rivals in Europe and the big Gulf-based airlines: Emirates, Etihad and Qatar Airways.

The airline said: “A special flight schedule for 25 November 2016 will be activated as soon as possible.”

Domestic passengers are being urged to travel by train instead. Passengers from Heathrow, Birmingham, Manchester and Edinburgh whose flights are cancelled may be re-routed on other airlines.

The strike is taking place at a time when there is a reasonable amount of capacity on other carriers. But there are concerns that the pilots may intensify their industrial action in the build-up to the busy Christmas season.