France's Michel Barnier faced an abrupt end to his premiership on Monday after key opposition parties said they would back a no-confidence motion against his government after only three months in power.

Certain that the National Assembly would deny him a majority backing the government's social security financing plan for next year, Barnier forced through the bill without a vote, using executive powers under article 49.3 of the French constitution.

The conservative premier, who formed a minority government in September after an inconclusive general election, has lived under the constant threat of a no-confidence vote that could force him to quit.

The government could be toppled as early as Wednesday, when a vote is expected.

Barnier called it "a moment of truth in which everybody must take their responsibilities", warning against lawmakers putting "partisan interests" before the "best interest of the nation".

Far-left opposition party LFI said immediately it would bring a no-confidence motion which the far-right National Rally (RN) - the largest single party in France's parliament - said it would vote in favour of, after accusing Barnier of failing to negotiate on some of the bill's provisions.

Key to any such vote is Marine Le Pen, the parliamentary leader of the RN that has opposed several parts of the government's 2025 budget plan, including the social security financing bill submitted to the assembly on Monday.

In a last-ditch concession to the RN, Barnier's office said it was scrapping plans for a less generous prescription drug reimbursement policy from next year.

But the nod to Le Pen's concerns was not enough to avert the no-confidence motion for Barnier, who has little hope of finding any left-wing support. (AFP)