She wore Prada, not Chanel, and was clearly not cut out for the role of France's first lady. But beyond the obvious, kind explanation for Cecilia Sarkozy's divorce lies a more intriguing question: why, in a land of such apparently relaxed sexual mores, was it important for President Nicolas Sarkozy (as well as for his also now separated Socialist rival, Segolene Royal) to appear to be in relationships as they raced towards last May's presidential election?

'The answer,' said writer Christine Clerc, 'lies in the fact that, contrary to what everyone outside France thinks, our republic is a monarchical system. The French first lady is better compared to Princess Diana than to some other President's or Prime Minister's wife,' said the author of Tigres et Tigresses, a history of French presidential couples. 'Her role, correctly played, is to be a compassionate bystander.'

Liberation columnist Laurent Joffrin said that, while Sarkozy had not had the courage to campaign for the presidency as a divorced man, last week's announcement proved him to be in step with the times: 'He is part of the "me" generation, not the "one" generation in which one's family life took second place to the nation's wellbeing.'

Last year 59 per cent of first-born children came into the world out of wedlock, putting French disenchantment with marriage second only to that of the liberal Swedes. Only 268,000 people got married in France in 2006 - the lowest figure since 1990. There is now more than one divorce for every two marriages. The 152,000 divorces last year marked an increase of 40,000 on 2002. Five times more French people got divorced last year than in 1963.

But the French are still getting together, albeit with fewer legal obligations than traditional town hall or church weddings imply. The Pacs (pacte civil de solidarite) - the partnership contract introduced in 1999 mainly with gays and lesbians in mind - is now more popular with heterosexuals than homosexuals. Ninety per cent of the 154,000 Pacs contracts signed last year - providing for joint taxation but conferring little legal security - were entered into by heterosexual couples. Even though Pacs partnerships can be ended simply by sending a recorded delivery letter to a judge, only one in seven of the contracts ends in 'divorce'.

Clerc says the Sarkozy divorce proves little else other than the fact that France's political class lags behind its people. 'There was a huge debate among politicians about introducing the Pacs, but when it came into law the people did not bat an eyelid,' she said.

Her book charts the moral dilemmas of a succession of French Presidents, including Francois Mitterrand, who was estranged from his wife, Danielle, well before he was elected in 1981 and was living partly with Anne Pingeot, with whom he had a daughter, Mazarine. Mitterrand was obsessed with keeping Mazarine's existence secret, to the extent of ordering phone taps on people who might reveal it. The media played the secrecy game for 13 years.

The first French politician to break the divorce taboo was Michel Rocard, who only recently remarried when he took up the job of Prime Minister in 1988. When Rocard's second marriage was on the rocks, Mitterrand advised him against getting divorced if he ever wanted to run for the Elysee Palace.

Similarly, it is often said that Jacques Chirac cut off an extramarital affair in the 1970s after advice from within his party that it was likely to threaten his chances of becoming President.

But there are also startling examples of modernity in the French political class. The straitlaced interior minister, Michele Alliot-Marie, has spent 23 unmarried years with Patrick Ollier, a member of the National Assembly. Had Royal been elected President, she would have entered the Elysee as the unmarried mother of four 'love children'. She spent 25 years with the father of her children, Socialist first secretary Francois Hollande, before separating from him in June this year.

Sarkozy clearly approached the announcement of his divorce with great unease, choosing Thursday's national strike as the day for issuing the 15-word communique. 'Clearly,' said Clerc, 'it is still, on balance, better for a President - or candidate - to present his or her relationship as an image of stability, reliability and faithfulness.

'On the other hand, we saw Cecilia fail to turn up for lunch with George Bush. We saw her cut short her visit to the G8 summit in Berlin. I think the French would rather have a divorced President than a first lady who cancels appointments.'