French Socialist party presidential candidate Benoît Hamon has called for the country’s divided left to join forces following the decision by former Socialist prime minister Manuel Valls to switch sides in next month’s election.

After weeks of speculation, Valls announced he was supporting Emmanuel Macron, breaking a signed agreement to back his party’s elected candidate.

The decision sparked anger and fear that it could spell the final nail in the coffin of the now bitterly divided Socialist party (PS).

Hamon, who polls suggest is set for a humiliating fifth place in the first round of the vote on 23 April, said the defection was “another great blow to democracy”.

Reading a statement on Wednesday, Hamon said: “Words that are given, that are signed before the people, must be scrupulously respected.”

He launched an appeal to “citizens of the left” – including his presidential rival, the hard left’s Jean-Luc Mélenchon – to rally and “turn the page on this old kind of politics, those politicians who no longer believe in anything and who go where the wind takes them, with little regard for their convictions”.

During the party primaries in January, Valls had pledged to support whoever became the PS’s presidential candidate. His announcement that he would now not support Hamon after all was seen by critics as a betrayal and a new low in a scandal-hit election.



Arnaud Montebourg, another former Socialist minister and primary candidate who immediately threw his support behind Hamon when the leftwinger became the nominee, was among the first to react to Valls’s defection.

In a virulent tweet, he wrote: “From now on everyone knows that an engagement made on one’s honour by a man like Manuel Valls is worth: nothing. And that’s what a man without honour is worth.”

Patrick Mennucci, a Socialist MP, wrote: “Manuel Valls you shame us.” Others contributed scathing responses accusing Valls of being “pathetic”, “a traitor”, and “a saboteur”.

Mathieu Hanotin, one of Hamon’s campaign directors, said Valls’s defection was “a pathetic attempt at sabotage”.



“I just want to say to Manuel Valls: it won’t work. The campaign isn’t finished. I always find it bizarre when political leaders shout about the wonderful things in our republic, about democracy, and all those great ideas and then, in the end, act only in their own interest,” Hanotin added.



He said Valls’s action was “the last appearance of a whole group of people, from the left and right, who want to continue to cling to power at whatever cost”.



After Hamon’s statement, Pascal Cherki, a socialist MP and Hamon supporter, told journalists: “This is about the central question of respecting one’s word. Manuel Valls has at least clarified things. He lied to the French ... now he has made the choice to leave the left.”

Cherki rejected suggestions it was the end of the PS and called on the left to reunite. “Together, the left can win this election. Divided it will have nothing,” he said.

Valls and Macron were both ministers in François Hollande’s government from 2014. Macron quit last year to prepare a presidential bid under his own political banner, En Marche! (“onwards”).

Hamon was already reeling from the decision of the popular defence minister Jean-Yves Le Drian, to support Macron. On Wednesday, he told France 2 television: “I’m not surprised. This sort of soap opera is meant to weaken me. I’m running my campaign by talking about the French’s daily life, not Valls’s life.”

Other Socialist party heavyweights have also defected to Macron, who is insisting his programme is neither right nor left or even centrist, but independent.

The support of Valls, a man deeply associated with the unpopular Hollande, could backfire on Macron.

Macron thanked Valls for his support on Tuesday but appeared to distance himself from the polemic. “I will be the guarantor of a renewal of faces, a renewal of practices,” he told Europe 1. He stressed he would not be naming Valls prime minister if he won the election.

Valls defended his decision on the news channel BFMTV, saying his first concern was for the country. “We mustn’t take any risk with the republic,” he said. “I have negotiated nothing and I am asking nothing. I’m not rallying, I’m taking a responsible position.”

Valls admitted he had gone back on his word to support Hamon. “It’s true, and that’s why I thought hard before agreeing [to speak]. However, the superior interest of France is more important than the rules of a party, a primary or a commission.”

Less than four weeks from the first round of the presidential election, polls suggest Macron would face the far-right Front National candidate Marine Le Pen in the second round.

François Fillon, the candidate for the rightwing Les Républicains party, has seen his own chances fall away on the back of a “fake jobs” scandal. Once tipped for victory, he has slipped back into third place over allegations he misused public funds with payments totalling €680,000 (£590,000) to his wife, Penelope.

On Tuesday, Penelope Fillon was charged with complicity in the abuse of public funds in the scandal.