PARIS -- French Prime Minister Francois Fillon resigned Saturday along with his government ahead of a long-planned Cabinet reshuffle.

The office of President Nicolas Sarkozy said in a two-line statement that the president accepted the resignation, "thus putting an end to Mr. Francois Fillon's functions."

The move was a formality because the prime minister must formally resign before a new government can be put in place. The president's office said there would be no more news Saturday, leaving the country hanging expectantly -- without a formal government.

It is unusual that a new government is not named on the same day the prime minister resigns -- even if he is reinstated in the reshuffle, as many observers believe Fillon may be.

"I think the president wants a few more hours to reflect ... This leaves the game completely open," Dominique Paille, spokesman for Sarkozy's conservative UMP party, said on BFM-TV.

It has been widely assumed the Cabinet changes would be announced Monday, but the evening resignation of Fillon and his government raised the possibility of an unusual Sunday announcement.

Constitutional expert Guy Carcassonne told France-Info radio that the just-resigned government is still able to get on with business in progress. But "generally ... one doesn't leave France without a prime minister for more time than is necessary."

Sarkozy said in June that he planned to change the Cabinet once a reform of the pension system was adopted. The hotly contested reform raising the retirement age from 60 to 62 became law Wednesday, a day after Sarkozy signed the measure, adopted by parliament Oct. 27. The reform triggered weeks of strikes around France, including at oil refineries and depots that put fuel-starved France on a crisis footing.

The strikes battered the image of the conservative Sarkozy who has had record low poll ratings despite the broad mandate to reform that he received when elected in 2007. It is assumed that in naming a new government he would try to create a solid base for 2012 presidential elections. He has not yet said whether he would seek a second term.

The popularity of Fillon, who has led the government since Sarkozy took office in May 2007, has consistently been several notches above that of Sarkozy despite his dry, businesslike aura.

France has had several "mini" Cabinet shuffles under Sarkozy, the latest in March. The biggest changes were made in June 2007, a month after the president took office, and was triggered by legislative election defeats.

Sarkozy's popularity has been at a steady low. A poll published Monday gave him a ranking of 35 percent. Fillon garnered a 48 percent positive ranking in the poll of 952 people by the LH2 firm. No margin of error was available but it would be plus or minus 3 percentage points for that number polled.