Marseille Socialist Party (PS) member Sylvie Lions-Noguier has filed a case for breach of trust against the party and the body that ran the primary election, in which left-winger Benoît Hamon beat Valls and five other candidates.
Following the vote "all [party] members were duty bound to support the elected member, in particular Manuel Valls", her complaint says, pointing out that all the candidates, including Valls, signed a pledge to back the winner.
Party officials seem reluctant to enforce that commitment, she has said on social media.
Valls calls for compromise with Fillon
Despite a storm of criticism in the media and from the Hamon camp, Valls's supporters were unapologetic on Thursday.
They will not be joining Macron's En Marche ! (Let's go!) movement but the Socialists should join a coalition formed by the former economy minister if he wins, Senator Didier Guillaume said.
MP Malek Boutih said that Hamon's fall to 10 percent in the latest opinion polls "signals the end of the PS" and called for a "change of name, practices and ways of joining".
Meanwhile, hard-left candidate Jean-Luc Mélenchon, who has risen to 15 percent in the latest poll, ruled out standing down in Hamon's favour at a 5,000-strong rally in the Channel port of Le Havre.
Fillon said he was, for his part, ready to work with the former Socialist prime minister, adding "I've always had a constructive dialogue with Manuel Valls" but criticising Macron, who has overtaken him in the opinion polls, for having "inspired" the policies of President François Hollande's government.
Macron defends EU
Macron himself was at a conference of farmers' union FNSEA on Thursday.
"We need, you need, Europe," he told them. "All the proposals that promise to separate you from Europe, to live outside Europe, are lying to you."
"The protection of our agriculture will be through Europe," he added.
As well as Fillon, who is pro-EU, several Eurosceptic candidates, including the far right's Marine Le Pen, are to address the meeting.