More than four-fifths of the 74 percent of pilots who took part in the ballot agreed to pursue the industrial action beyond the current deadline of Monday, said Jean-Louis Barber, head of the Air France section of the SNPL.

"It could continue even further (beyond Sept. 26), given the very strong mandate," Barber added, calling for a meeting with French Prime Minister Manuel Valls to help resolve the conflict with management.

Air France said on Friday it expected to operate 45 percent of its flights on Saturday, based on an estimated 60 percent of the pilots walking out.

The situation is expected to worsen on Sunday, with just 38 percent of flights going ahead - the lowest level since the strike began on Monday - and 65 percent of pilots walking out, the airline said on Saturday.

The pilots are protesting over Air France's plans to expand the low-cost operations of its Transavia brand by setting up foreign bases as it seeks to fight back against fierce competition from budget carriers.

The expansion of Transavia is part of a new strategic plan unveiled this month aimed at boosting earnings. The proposals would see Transavia's fleet rise to 100 jets by 2017, from about 50 now, and the number of passengers more than double to 20 million.

JOB FEARS

The company has said the idea is not to replace Air France but to complete its armoury to attack the leisure market.

But the SNPL said it was concerned that Air France would abandon Transavia's development in France altogether, blaming it on pilot opposition, and then focus on the unit's expansion elsewhere in Europe, thus moving jobs outside the country.

"The SNPL, which wants to develop Transavia France, ... today feels betrayed by management," Barber said, adding that it was clear Air France wanted to shift jobs outside France through Transavia Europe.

"We call on the prime minister to hold a meeting quickly, and we have no doubt that he will pay close attention faced with the plan to relocate our jobs."

Air France, which has said the strike is costing it 10 million to 15 million euros (7.8-11.7 million pound) a day, deplored the move to continue the strike.

"Air France has maintained constant dialogue with its pilots in order to reach an agreement to benefit the group's growth and competitiveness," Air France Chief Executive Frederic Gagey said in a statement on Friday after the union gave notice it may extend the strike.

"I am sorry to announce that our concrete proposals to reassure our pilots have not yet drawn a reasonable response."

Shares in Air France-KLM, the Franco-Dutch parent group, have lost 5.7 percent since the strike began, ending 2.5 percent lower on Friday.

(Reporting by Gregory Blachier and James Regan; Editing by Gareth Jones)

By James Regan and Gregory Blachier