The left beat the ruling conservatives in almost all of France's 26 regions on Sunday, raising doubts about the government's commitment to push through ambitious economic reforms such as cost-cutting in the public health system.
The Socialist Party and its allies, buoyed by discontent with the government's policies, won about 50% of the votes and at least 21 of the regions, exit polls showed.
The centre-right won around 37% and the far-right about 14% of the votes in the second round run-offs.
"The French people have first of all sent a heavy vote of sanction to the entire government of Jean-Pierre Raffarin," said Socialist Party leader Francois Hollande.
The left had been expected to win after making gains in the first round of voting on 21 March but the extent of its victory was a shock, painting France's regional political map almost entirely red.
Chirac did not comment on the results. But Raffarin, looking pale, went on national television to say the government would not abandon reforms intended to cut the soaring public finance deficit in the eurozone's second largest economy.
Economic analysts said Raffarin could be on the way out and that the crushing defeat was likely to weaken the government's resolve to go as far or as fast as planned in reforms.
Chirac's centre-right UMP party had controlled 14 regions since the last regional vote in 1998 and the left had held 11.
The left made a big rebound from the 2002 parliamentary election, when it lost power with 37% of votes compared with the centre-right's 43%.