Former French Socialist prime minister Manuel Valls has said he wanted to stand for President-elect Emmanuel Macron's political movement in June parliamentary elections.
It is the first high-profile defection since Mr Macron's election win on Sunday.
Mr Valls' move could be a boost for Mr Macron, who needs to secure a majority in the elections to have a realistic chance of implementing his ambitious reform plans.
But Mr Macron will be cautious about inviting too many prominent former Socialists into his movement as that would lend credence to the arguments of his conservative opponents that Mr Macron's administration will be a continuation of outgoing President Francois Hollande's unpopular administration.
Party spokesman Benjamin Griveaux said Mr Valls had not applied to the party's selection committee yet and had 24 hours left to do so.
"I will be a candidate for the presidential majority and I wish to join his (Macron's) movement," Mr Valls, who was prime minister in Mr Hollande's administration between 2014 and 2016, told RTL radio.
"This Socialist party is dead. It is behind us," he added.
"The essential thing today is to give a broad and coherent majority ... to Emmanuel Macron to allow him to govern."
The defection highlights the disarray in the Socialist party, whose candidate Benoit Hamon attracted just 6% of votes in the first round of the presidential election.
Mr Valls, who announced in March he would vote for Mr Macron in the presidential election, is on the right of the Socialist party and has similar pro-business views to Mr Macron, who will assume office next Sunday as France's youngest leader since Napoleon.
Mr Macron's party currently has no seats in parliament.
An opinion poll last week predicted, however, that his party is set to emerge as the largest in the parliamentary elections.
Mr Macron's party has announced that the 'En Marche!' movement will now change its name to 'En Marche la Rpublique' or 'Republic on the Move', so as to structure itself more like a traditional party.
The names of Mr Macron's 577 candidates in the legislative elections are to be announced on Thursday.
