France's former Socialist Prime Minister, Manuel Valls, has said that he will support President-elect Emmanuel Macron's start-up party – now named La République En Marche! – in next month's parliamentary elections.

Macron secured an emphatic victory over far-right politician Marine Le Pen on Sunday, but now faces the challenge of getting a majority in parliament so that he can implement his plans for economic recovery.

His party was only created 13 months ago and does not have any representatives in the National Assembly.

Valls, who resigned as prime minister in December after he failed to be win the Socialist nomination for president, backed Macron in the race in March and has now promised to abandon the socialists and sign up to centrist La République En Marche.

"I will be a candidate in the presidential majority and I wish to join up to [Macron's] movement," Valls, who led the centrist branch of the Socialist party, told RTL radio.

"I am attached to the Socialist party, its history, its values but the Socialist party is dead and gone," he said.

Macron, who served alongside Valls in Hollande's cabinet, has to submit his list of candidates on 11 May. Benjamin Griveaux, a spokesman for the president-elect, told French radio that Valls had not yet submitted his application online to join the party.

"He should have applied like everyone else because the rules are the same for everyone. If you don't put your name forward you can't be selected by En Marche. He's got 24 hours," he said.

Valls' comment that the Socialist party is "dead and gone" provoked anger from his colleagues.

"I hope for France's sake that Brutus and Judas don't seek En Marche's party nomination," Socialist MP Alexis Bachelay wrote on Twitter.

But EU commissioner Pierre Moscovici, who was finance minister under Hollande from 2012-14, urged his party not to establish itself as Macron's opposition.

"There's no sense for a Socialist to work against Emmanuel Macron. He's pro-European, he has progressive ideas and he hasn't unveiled all of his plans nor named his government," he told journalists.

Macron and Valls have not always been allies. In a fly-on-the-wall documentary shot during Macron's election campaign, the president-elect described Valls' incendiary comments about President Hollande when both men were considering running in the primary as a "real betrayal."

"If there is traitor, someone who has pulled the trigger on Hollande, it's Valls," Macron says in the Behind the Scenes of a Victory documentary which aired on 8 May.