France's leftwing parties will reveal the political manifesto for their renewed alliance on Friday, which politicians from the bloc said included policies that would lower the retirement age, link salaries to inflation and introduce a wealth tax for the rich.

Political parties including the Socialists, Greens, Communists and hard-left Unbowed France (LFI) will form a "Popular Front" in a bid to challenge the far-right's National Rally (RN), leading the polls for upcoming snap parliamentary elections.

"Warmest congratulations and thanks to our negotiators who had four sleepless nights at work, (going through) the program line by line, constituency by constituency," French far-left leader Jean-Luc Melenchon said on social media platform X.

Ian Brossat, a communist member of the Senate, said the final deal, which will be presented during a news conference midday, includes the withdrawal of President Emmanuel Macron's unpopular pensions reform which led to protest on the streets.

"There will be the withdrawal of the pensions reform and a return to 60 years (as retirement age)", Brossat told Public Senat television. Macron's reform included a gradual rise of the retirement age to 64 from 62 before the overhaul.

Although the left's chances of winning the elections are slim according to the polls, their tie-up could bundle enough votes to hinder both Macron's and Le Pen's camps from reaching a stable governing majority, handing it sizeable political leeway.

The bloc worked together during the previous parliamentary campaign in 2022 before a leadership struggle and policy differences - including on the Gaza war - led to the de-facto collapse of their alliance.

Brossat said the parties managed to agree on a common stance on the conflict in the Middle East, labelling Hamas a "terrorist organisation" while also calling for the recognition of a Palestinian state.