France’s former Economy Minister Emmanuel Macron looks set to unveil his bid for the presidency.
He is not expected to join the race to be the Socialist candidate, but as head of the centrist En Marche movement he created in April.
The move has unsettled Republican rivals vying to be selected in new US-style primaries from Sunday.
Frontrunner Alain Juppe said Mr Macron had betrayed President Hollande, “whom he’s stabbed in the back”.
The former economy minister is a one-time protege of the president who quit the unpopular Socialist government in August amid discontent within the party over his political ambitions.
Mr Hollande has not yet confirmed whether he will stand for re-election and will make a decision in the coming weeks. He has the lowest popularity ratings of any post-war French president.
Mr Macron, who has never been elected, was expected to end speculation at a press conference in a Paris suburb.
He launched En Marche in April to promote “new ideas… neither of the right nor the left”. The movement now has almost 97,000 members and has already received €2.7m ($2.9m; £2.3m) in donations.
It is thought any presidential bid will aim to exploit the deep divisions in the French left – with one survey suggesting that 38% of French voters believe Mr Macron would be a good president.
But it has also caused upset among some of the centre-right candidates vying for the Republican nomination – which, for the first time, will be chosen ahead of the presidential vote in April and May, in a nationwide vote of supporters in a US-style primary election beginning this Sunday.
The favourite to win the poll, running on a centrist ticket, had been former Prime Minister Alain Juppe.
But he is facing stiff competition from former President Nicolas Sarkozy, who is running a law-and-order campaign, and Thatcherite former PM Francois Fillon.
The winner of the Republican race is seen as most likely to take on National Front (FN) leader Marine le Pen in a second-round presidential runoff.
Mr Sarkozy’s bid was dealt a blow on Tuesday after new allegations emerged that he received millions of euros in illegal financing from the regime of late Libyan dictator Muammar Gaddafi.
He and his former chief of staff have denied wrongdoing in the case, which involves funding for his successful 2007 presidential campaign.
Meanwhile, President Francois Hollande has said France’s state of emergency should continue until the election.
It was due to be lifted in January, having been in place since the November 2015 terror attacks in Paris. Mr Hollande’s proposal would see it kept until 7 May, the second and final round of the presidential election.
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