Mr Valls's decision is infuriating some Socialists, already deeply divided over the legacy of unpopular president Francois Hollande's term.
Hamon, a hardline Socialist who wants to legalize cannabis and create a basic monthly state income for all, has struggled to win support from center-left Socialists like former prime minister Manuel Valls who said on Wednesday he would vote for Macron.
Mr Valls considers the independent as the best candidate to face far-right leader Marine le Pen.
It also raised awkward questions about Mr Macron's ability to obtain a parliamentary majority in the legislative elections that will follow the presidential poll.
Socialist Party chief Jean-Christophe Cambadelis said he was "sad" he couldn't stop Valls endorsing Macron, but warned against an internal purge, saying it could hurt Socialist candidate Benoit Hamon.
"I have to say I have taken part in a few elections but I have never seen an executive, just days out from the election, clinging on to its privileges" and influence, former prime minister Mr Fillon told a rally in the western city of Nantes.
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".
BlackRock reveals major overhaul of active equity offering
BlackRock's equity overhaul also invites comparisons to that of another major asset firm rival, Pacific Investment Management Co. So to combat their woes, BlackRock is shifting to quantitative strategies like many fundamental hedge funds are.
Mr Macron raised concerns over the affect Brexit would have on French nationals living in London but added: "Obviously we will work together with the United Kingdom and with London because they live there". Once the front-runner in the race for the April 23-May 7 election, Fillon is presenting a new version of his campaign platform Monday in an effort to claw back support.
Centrist frontrunner Emmanuel Macron brushed off concerns about his ability to form a parliamentary majority if elected, declaring his fledgling En Marche (On the move) party to be "France's premier political force".
Parliamentary elections take place across France in June to choose lawmakers.
Mr Macron wants to present a candidate from his own movement, En Marche (In Motion), in every district. "So, I will vote for Emmanuel Macron".
Macron is now the favorite to win the election.
PARIS-French presidential hopeful Emmanual Macron on Tuesday defended his "method of real shifts in power" and specified the manner in which he plans to run the country if he gets elected.