French far-right leader Marine Le Pen has delivered a further blow to President Nicolas Sarkozy's hopes of re-election by refusing to endorse him and telling her six million supporters to make their own choice at Sunday's ballot.

Mr Sarkozy, who faces off against Socialist Francois Hollande on May 6, needs many of the 17.9% of voters who chose National Front leader Ms Le Pen last week to back him in the run-off if he is to overcome first-round winner Mr Hollande.

However, Ms Le Pen, who came third last month with a result that eclipsed her father's record at the head of the populist protest movement, told a rally in Paris yesterday she would personally spoil her ballot paper.

She told supporters at an annual commemoration of national saint Joan of Arc: "I will not grant my trust, or a mandate, to these two candidates. On Sunday, I will cast a blank ballot."

Ms Le Pen did not further twist the knife for the conservative incumbent by urging her 6.4 million supporters to vote likewise.

However, in not directing her supporters, it is unclear how many will now stay at home and how many will turn out to vote.

Mr Hollande has a six-to 10 percentage point lead in opinion polls.

Ms Le Pen said: "I have made my choice. Each of you will make yours."

Analysts had calculated Mr Sarkozy may need as many as 80% of Ms Le Pen's first-round voters if he were to win, but polls indicate that only half intend to switch.

With a parliamentary election to come in June, National Front leaders believe they can break through and win seats in the legislature, especially if a heavy defeat for Mr Sarkozy plunges his centre-right UMP party into deeper disarray.

Being punished for economic gloom, rife unemployment and a widespread dislike of his presidential manner, Mr Sarkozy is the most unpopular sitting president to run for re-election and the first in the 54 years of the current electoral system to lose a first-round vote to a challenger.

Mr Hollande, a mild-mannered centre-leftist running on a tax-and-spend platform, would be the first left-wing president in 17 years to lead the eurozone's second-biggest economy. He made his own campaign speech in central France, where he visited the tomb of Socialist former prime minister Pierre Beregovoy, who shot himself on May 1, 1993.

He criticised Mr Sarkozy for trying to distract attention from labour day marches, where senior Socialists are guests of honour.

He said: "May 1 is a celebration of the unions.

"I can't accept that in France on May 1 there is a battle against unionism."

Mr Sarkozy said earlier the left had no exclusive rights over May Day.

He said: "I didn't realise that May Day was reserved for the union struggle.

"It's to celebrate work, which is a founding value of our society."

Ms Le Pen's surprise result in the first round has thrown the last days of the election race into disarray, with Mr Sarkozy striving to court her voters without alienating the centrists he also needs.

He said: "I need the centrist voters and those who abstained just as much as I need those who voted for Marine Le Pen in the first round."