French Prime Minister Jean Castex says his government will tender its resignation if Emmanuel Macron is re-elected president, in a bid to provide a "new impetus" for Mr Macron's centrist party ahead of parliamentary elections in June.

Mr Macron is facing a close race in the run-off on Sunday against far-right veteran Marine Le Pen, with analysts warning that low turnout could prove a wildcard even though most polls show Mr Macron in the lead.

If he is re-elected, "in the subsequent days, as is the tradition, I will present the president with my resignation and that of the government," Mr Castex told France Inter radio.

"I'm among those who think a new impetus should be found after the president's re-election," he added.

The parliamentary vote in June will be a further test of Macron's popularity, and renewing his majority will be essential for pursuing his reformist agenda, including an overhaul of the pensions system that would require most people to work longer before retiring.

There has been speculation that European Central Bank chief Christine Lagarde could be offered the post of prime minister, although she sidestepped a question on the issue at a press conference last week.

Opinion polls currently put the centrist Macron at 53-56% in the run-off against 44-47% for Le Pen, in a much tighter finish than in 2017, when the same candidates faced-off but Mr Macron carried the vote with 66%.

"The game isn't over yet and we certainly can't draw conclusions ... that this election, this match, is already decided," Mr Castex said.

"We have to convince the French that Emmanuel Macron's programmes are the best for France and for them."

Mr Macron and Ms Le Pen will face off tomorrow evening in their first televised debate of the campaign, which could prove critical for winning over millions of voters, in particular on the left, who have told polling firms they remain undecided.