By Michel Rose and Sudip Kar-Gupta
PARIS (Reuters) - Former Prime Minister Manuel Valls said on Wednesday he would vote for Emmanuel Macron in France's presidential election, becoming the biggest Socialist Party name to turn his back on its official candidate and support the centrist instead.
While it was not clear if Valls' defection would benefit poll favorite Macron, who politely thanked Valls, it prompted angry responses from many Socialists and media speculation about the survival of the largest left-wing party.
Valls, whose announcement came days after veteran Socialist defense minister Jean-Yves Le Drian deserted to Macron, said he wanted to do all he could to ensure that far-right leader Marine Le Pen, second-placed in opinion polls, did not win power.
"I'm not going to take any risks," Valls said, adding that he believed Le Pen's score potential was seriously underrated. "I will vote for Emmanuel Macron," he told BFM TV.
French opinion polls show Macron winning the presidency in a second-round vote on May 7 where he would face off against Le Pen. They show Socialist Benoit Hamon set for a humiliating fifth place in the first round eliminator on April 23.
Hamon, a hardline Socialist who wants to legalize cannabis and create a monthly state payment for all, is on course to win only 10 percent of the vote in the first round, according to an Elabe poll published on Wednesday.
Hamon denounced Valls' defection and called on all left-wingers to unite behind him 25 days from round one of the election. "I urge you to sanction those who've started this morbid game...those who no longer believe in anything," he said in a statement.
Valls said his choice did not mean he would campaign for the 39-year-old Macron - a fellow minister in President Francois Hollande's government from 2014, but who quit last year to prepare a presidential bid under his own political banner En Marche! (Onwards!).
Valls, who lost to radical left-winger Hamon in the Socialist primaries, is seen by political sources and experts as likely to wait in the wings and seek to build a reformist parliamentary force that would be distinct from En Marche!, but which could get a say in its parliamentary majority should Macron become president.
"I have nothing to negotiate and am not asking for anything, I'm not joining his camp," Valls said. "But nothing will be the same after this presidential election...The duty of reformists is to play their part in a governing parliamentary majority."
Macron, who has drawn support from the political right as well as left, was also quick to say he did not plan to bring Valls into his government. "I shall be the guarantor of new faces, new ways of doing things," he said on Europe 1 Radio.
The news came a day after third-placed candidate Francois Fillon, under formal judicial investigation on suspicion of financial impropriety, suffered a further blow when his British wife Penelope was put under formal investigation as well.
The inquiry centers on allegations that the couple misused hundreds of thousands of euros in public funds, with him paying her a lavish tax-funded salary for minimal work as his parliamentary assistant.
Francois Fillon has conceded what he called errors of judgment but denies doing anything illegal.
Valls' endorsement is a mixed blessing for Macron, even though their political views are not far apart.
Fillon, who has promised to slash government spending, seized on Valls' move to say their would be no break with the past under Macron as both men were key ministers under Hollande.
"All of Hollande's team is backing Emmanuel Macron. It's as I've always said, Emmanuel Macron is Francois Hollande," Fillon told reporters.
For many Socialists, and above all candidate Hamon, Valls' decision comes from a man who represents Hollande's rightward turn during his five-year mandate toward the business-friendly reforms that upset the left and alienated core voters.
The blow for Hamon clearly compounded existing left-right splits within the party.
"Everybody now knows what a commitment signed by a man like Manuel Valls is worth," Arnaud Montebourg, a more hardline leftist in the Socialist Party, said on Twitter.
(Reporting by Sudip Kar-Gupta, Michel Rose and Leigh Thomas; writing by Andrew Callus and Brian Love editing by Mark Heinrich)