Valls resigned as prime minister to launch his own presidential bid but in a shock result was defeated in the Socialist Party's primary in January by left-wing candidate Benoit Hamon.
The centrist Macron, 39, formed his own party only last year, and his election victory over the National Front's Marine Le Pen on Sunday has destroyed the dominance of the centre-left and centre-right parties which have governed France for almost 60 years. To compete, "en Marche" has rebranded itself is now rebranding itself "La Republique en Marche" (Republic on the Move), as it prepares a list of candidates.
So far the names of only 14 candidates are known, but on Thursday afternoon "around 450" will be revealed, Jean-Paul Delevoye of Republic on the Move said. "(Vall's) voice is not insignificant, but his candidacy will be treated like anyone else's".
Hamon, who gained popularity in recent years by leading a group of rebel Socialist lawmakers who opposed Valls, was a distant fifth in the first round of the presidential election after garnering just over 6 percent of the votes, the Socialist Party's worst result since 1969.
Mr Macron won the presidency with 66 per cent of the votes cast for a candidate, but the election also had a high number of blank or spoiled votes and an unusually low turnout.
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Commentators were already focused on Macron's next battle as part of his planned "revolution" of French politics: parliamentary elections on June 11 and 18.
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If Macron's party performs poorly, he could also be forced to form a coalition government, a common occurrence in many European countries but something very unusual in France. If another party wins a majority, Macron could be pressured to choose a prime minister from that party. But for the first time in the country's post-war history, the new President does not have a big party machine behind him, with the two main governing parties, the Republicans and Socialists, crashing out in the first round.
Sylvie Corbet in Paris contributed.