The walkout at Lufthansa brands Germanwings, Eurowings, Lufthansa City Line and Sunexpress had initially been scheduled for 0300-0900 GMT but the union in a statement on Sunday said industrial action would now be extended until midnight local time (2200 GMT), citing threats made by the airline over jobs as reason.

A Lufthansa spokesman said the strike was illegal because UFO's status as a negotiator on behalf of staff was in doubt.

Lufthansa on Friday had offered a 2% pay rise to cabin staff, prompting the union to call off a planned strike at Lufthansa's namesake core brand at hubs Frankfurt and Munich.

But the dispute deteriorated after UFO found the offer lacked concessions on expenses and employment conditions.

The cabin crew union has for months fought with the airline in court over UFO's legal status. Lufthansa claims the union's new leadership team that took office earlier this year was not elected in a way that met legal requirements.

(Reporting by Ludwig Burger and Elke Ahlswede. Editing by Jane Merriman)