By Elizabeth Pineau

PARIS (Reuters) -France's Socialists filed a no-confidence measure against Prime Minister Francois Bayrou after pension talks collapsed, a senior party lawmaker said on Tuesday, but it seems unlikely to pass after the far-right indicated it would not follow suit.

Months-long talks between French trade unions and employers over reforms to the pension system crumbled late on Monday, prompting Bayrou to summon both sides for talks to find a way forward.

The failure of those talks makes Bayrou vulnerable, as his centrist government, grouping President Emmanuel Macron's Ensemble alliance and the conservative Republicans, could fall anytime if left-wing and far-right parties band together to back a motion of no-confidence.

It also augurs badly for what are likely to be even tougher talks over the 2026 budget bill, with the government seeking to push through 40 billion euros ($46 billion) in spending cuts.

Boris Vallaud, leader of the Socialists in the lower house, told parliament that Bayrou had not kept his promise to put a new pension reform bill to parliament.

"This compels us to file a motion of censure," Vallaud said.

The hard-left France Unbowed has previously supported the idea of a confidence vote, but without support from Marine Le Pen's far-right National Rally (RN), the measure looks unlikely to pass.

Gaëtan Dussausaye, an RN lawmaker, said the RN was not looking to topple the government, and Le Pen did not mention such a move when she addressed parliament.

"We don't censure for nothing — but that's exactly what the left is proposing here," said Dussausaye.

Bayrou finds himself in a similar situation to the previous prime minister, Michel Barnier, whose three-month-long administration was propped up by the RN until it backed a no-confidence measure in December over his belt-tightening 2025 budget bill.

In parliament on Tuesday, Bayrou said he was still convinced that "there is a path, albeit a very difficult one, that can lead us out of this impasse".

In what were billed as last-chance talks, pension negotiators failed to agree on how to amend an unpopular 2023 overhaul of the pension system that will gradually raise the retirement age to 64 from 62.

Unions wanted to allow workers with physically taxing jobs to retire earlier, and to give more weight to maternity leave, while employers were wary of concessions that could weigh on the system's finances.

Bayrou, a centrist and long-time debt hawk, had called the union-employer talks in a bid to prevent the Socialists withdrawing their support for him.

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(Reporting by Makini Brice, Michel Rose, Leigh Thomas and Gabriel Stargardter; Editing by Rachna Uppal and Kevin Liffey)