President Nicolas Sarkozy's centre-right UMP party suffered a heavy defeat in French regional elections at the weekend.
With more than 97% of the votes counted, the Socialist-led opposition alliance had 52% of the vote to the UMP's 35%.
Prime Minister Francois Fillon acknowledged the defeat, admitting the left's "success", and is expected to offer his resignation.
The BBC reports that Mr Sarkozy is unlikely to accept it, as the president's own approval ratings are at an all-time low. But his chief of staff says he's planning a "modest reshuffle" of the government.
The results leave the UMP in control of only one of France's 22 regions, the Alsace region in the east.
These elections are the last major electoral test in France before the presidential election in 2012.