France's former prime minister Manuel Valls endorsed the presidential bid of centrist Emmanuel Macron on Wednesday, becoming the most high-profile Socialist to back the ex-banker over the party's own nominee.
His support was seen as a mixed blessing for Macron, giving him the backing of a leading politician but weakening his claim to represent a break with the past.
Macron quit the Socialist government last year to form his own movement, En Marche (On the Move), saying he wanted to shake up the political class.
Valls' nod comes less than a month before the first round of the election on April 23, in which polls show Macron and far-right leader Marine Le Pen running neck-and-neck.
The former premier cited the "risks" of a Le Pen victory in confirming his support for Macron, in an interview with BFM TV.
Le Pen pounced on the defection as proof of her claim that Macron is effectively a surrogate for outgoing President Francois Hollande.
- 'Macron is Francois Hollande'-
It showed Macron was a "simple cog in a grand plan to save Hollande's entourage", the National Front leader wrote on her website.
Conservative candidate Francois Fillon put it more simply. "Emmanuel Macron is Francois Hollande," he said.
Macron, 39, thanked Valls for his support but said it did not mean the ex-premier would land a job in his cabinet if he won, stressing that his goal was to "refresh the faces" of French politics.
He has said his door is open to all who want to join his economically liberal pro-EU movement -- but that he will not be beholden to big-name endorsers.
Valls' choice of Macron is a further blow for the struggling Socialist candidate Benoit Hamon, 49, who beat Valls to the party's nomination in a January primary.
Asked how he would vote, Valls said he would back his former economy minister "because I think you should not take any risks for the Republic. So, I will vote for Emmanuel Macron."
His remarks were seen as a reference to a possible win by 48-year-old Le Pen -- a prospect that the upstart Macron, a relative newcomer to politics, is seen as best placed to prevent.
Macron is currently tipped to easily beat the anti-immigration, anti-EU Le Pen in the second round election run-off on May 7, but after Britain's shock vote to quit the EU and Donald Trump's surprise election in the United States no one is ruling out another upset.
- 'Stabbed in the back' -
Hamon won the Socialist primary on a tide of disaffection with the party's centrist lurch under Hollande and Valls.
But he has failed since to rally party heavyweights around his bid. On Wednesday evening, Hamon asked voters "to punish those who are part of this morbid game" and "turn your backs on these politicians who no longer believe in anything."
Defence Minister Jean-Yves Le Drian has also come out in favour of Macron, as has Lyon Mayor Gerard Collomb.
Hamon, who has fallen from fourth to fifth place in polls for the first round behind Communist-backed firebrand Jean-Luc Melenchon, has accused them of having "stabbed him in the back".
Political analyst Pascal Perrineau said the split was proof of Valls' prediction of a division into "two irreconcilable lefts" -- one that is still wedded to the class struggle and another that backs reforms.
Candidates from all the big traditional parties have seen their campaigns upended by the youthful Macron -- a fresh face in staid French politics -- and the nationalist Le Pen.
While drawing support from the left Macron has also won over some politicians on the centre and the right turned off by the multiple scandals engulfing Fillon, 63.
The Republicans candidate, once the favourite to become France's next leader, has seen his chances founder over an expenses scandal and revelations of luxury gifts from the rich.
Charged with misuse of public funds over payments totalling 680,000 euros ($739,000) to his wife for a suspected fake job as a parliamentary aide, the 63-year-old has slipped back into third place.
On Tuesday, his Welsh-born wife Penelope was charged with complicity in the abuse of public funds over the scandal.