French Prime Minister Manuel Valls announced Monday his candidacy in next year's presidential election, and will step down to focus on his campaign.
Valls had previously been expected to announce entering the race to be the Socialist candidate, after Francois Hollande said he would not stand, a move that would mean a government reshuffle.
Valls has called a news conference for 1730 GMT on Monday, and even before any announcement, French media were citing the health, interior and finance ministers as possible heads of what would effectively be a caretaker government if Valls stands down to focus on the presidential election.
Valls, 54, has made no secret of his desire to seek the Socialist ticket, even though he faces stiff competition from the more traditional left of his party at the primary in January. The prime minister has a more centrist, business-friendly stance on economic policies, combined with a reputation as a hardliner on law and order.
Surveys suggest he would win the Socialist Party ticket, but faces a tough fight against left-wing firebrand Arnaud Montebourg, one of several people taking part in the primary.
But opinion polls show the presidential contest will come down to a run-off on May 7 between right-winger Francois Fillon of the Les Republicains party and Marine Le Pen, leader of the anti-EU, anti-immigrant National Front Party.
Valls was due to lunch with Hollande, who, after almost five years in power marked by high unemployment and deadly Islamist attacks, said last Thursday he would not seek re-election.
Valls' office said he would speak in the town of Evry, his political fiefdom a short drive south of Paris.
French media said Valls would have to announce his resignation as prime minister, with Finance Minister Michel Sapin, Interior Minister Bernard Cazeneuve and Health Minister Marisol Tourine seen possibly replacing him.
Farm Minister Stephane Le Foll put his name into the hat in a radio interview, telling France Info state radio he was one of just three ministers who had been in government since the outset of Hollande's tenure in mid-2012.
The presidential election takes place in two rounds in April and May.