French unions are staging on Tuesday a 14th day of protests against government plans to raise the retirement age to 64, in what could be a final attempt to pressure lawmakers into scrapping a law that is already on the statute books.

President Emmanuel Macron's decision to force the reform through with special constitutional powers prompted angry protests this spring, but the issue has slowly moved down the media agenda, making it harder for unions to mobilise.

"Protests have been going on for six months, it's unprecedented," Sophie Binet, the new leader of the hardline CGT union said on BFM TV. "There's a lot of anger but also fatigue," she said, adding that strikers were feeling the pinch on paychecks.

Binet nevertheless banked on an "extremely high" level of mobilisation on Tuesday and said the CGT union was prepared to keep up the fight against the reform in the coming weeks.

Macron is now enjoying a timid rebound in opinion polls, having launched a PR blitz after the reform passed that saw him criss-cross the country to confront public anger but also to announce big investments in new technologies.

Between 400,000 and 600,000 people are expected to turn out at protests across France, authorities said, which would be down from more than a million who took part in marches at the height of the pension protests earlier this year.

Inter-city trains are likely to be only "slightly disrupted", the SNCF railway company said, while the metro network in Paris will run a normal service. One-third of flights out of Paris-Orly airport have been cancelled, however.

"This is likely to be one of the last days of protests againt the reform," Laurent Berger, the leader of the more moderate CFDT union told Europe 1 radio.

The CFDT must now turn anger against the reform into a "show of strength" in future talks with the government on issues such as improving work conditions to purchasing power, he said.

The unions, which have kept a rare united front during the whole pension episode, are holding the nationwide strike just two days before an opposition-sponsored bill aimed at cancelling the minimum pension age increase is reviewed by parliament.

The provision is expected to be rejected by the lower house's speaker, a member of Macron's party, because under the French constitution, lawmakers can’t pass legislation that weighs on public finances without measures to offset those costs.

But unions hope a big protest turnout could pressure lawmakers into reviewing the bill anyway and holding a vote. Opposition lawmakers, meanwhile, say the bill being rejected would revive public anger, branding any such move "antidemocratic".

Macron, who says the reform is essential to plug a massive deficit, will be hoping that the approaching summer holidays and improving inflation numbers will help the public move on.

The president's popularity has gained four points in a monthly Elabe poll in June and eight points in a YouGov poll, although it is still languishing around 30%.