French Prime Minister Manuel Valls will announce on Monday he is running to become the Socialist presidential candidate in elections next year, sources close to him told AFP.
The prime minister's office announced only that he would make a statement at 6:30 pm (1730 GMT) from his political base in Evry, south of Paris.
But his entourage confirmed the 54-year-old Spanish-born premier would end the speculation over whether he would run -- seen as highly likely following President Francois Hollande's announcement last week that he will not be a candidate.
Valls was to have lunch with Hollande on Monday, as usual, ahead of his speech.
The French left, in search of a new leader after Hollande's exit, will hold a two-round primary in January to choose a presidential candidate.
Polls show the far-right Marine Le Pen could triumph in the first round, but would likely lose to the rightwing candidate Francois Fillon in the run-off.
But with Sunday's Italian referendum defeat for Prime Minister Matteo Renzi just the latest rout of mainstream political figures at the ballot box, no-one is ruling out a Le Pen victory.
The divided French left currently appears ill-equipped for such a fight.
Polls currently show Valls would win the leftwing primary but would trail behind Le Pen and Fillon in the first round of the presidential election.
Valls would face a fight for votes with two independent candidates who are not standing in the primary -- business-friendly former economy minister Emmanuel Macron and the fiery hard-left Jean-Luc Melenchon.
If Valls does announce a run for the presidential office, he would have to resign as prime minister, sparking a government reshuffle.
Health Minister Marisol Touraine and Interior Minister Bernard Cazeneuve are two names tipped to succeed him as premier for the six months until legislative elections that follow the presidential vote.
Valls sought the Socialist nomination five years ago as a rank outsider, winning only 5.6 percent on a platform seen as too economically liberal.
He became a spokesman for Hollande's campaign and when the Socialist won the presidency he rewarded Valls with the post of interior minister.
In 2014, Valls was promoted to prime minister with a mandate to rein in a group of unruly ministers that were undermining Hollande's authority.
Within months he had a rival for the title of reformer-in-chief, in the telegenic banker-turned-economy minister Macron.
Hollande's protege walked out on the government in August to further his own presidential ambitions -- infuriating Valls who accused him of "destroying the left".
Valls said Friday he was determined to "defend the legacy" of Hollande, despite opinion polls showing that his former boss is the least popular French president for nearly 60 years.
But in a newspaper interview last month, the normally loyal Valls had himself turned on Hollande, saying explosive revelations contained in a book of interviews between the president and two journalists had "plunged the left into total disarray".
Valls said he wanted to "dispel the notion that defeat is inevitable" for the Socialists.