The shadow of sleaze has returned to haunt President Sarkozy, with prosecutors disclosing today that police are investigating claims that a property developer knocked at least 300,000 euros (£210,000) off the price of his family flat.

Officers of the Financial Crime Brigade have opened a preliminary inquiry into the terms of his purchase in 1997 of a six-bedroom, two-storey flat on the Ile de Jatte, a luxurious district in the suburb of Neuilly. Mr Sarkozy was mayor of the town, which adjoins western Paris, until his election in the spring.

The case has been opened on the basis of a citizen's complaint six months after questions emerged over the apartment deal during the presidential campaign. Mr Sarkozy denied at the time that there was anything shady about the purchase and blamed enemies of seeking to smear him. His spokesman declined comment today.

In February, Le Canard Enchaîné, an investigative weekly, reported that Mr Sarkozy and his wife, Cécilia, paid 875,000 euros for the new riverside apartment — which it said was 12-35 per cent below market price. It was built and sold by the Laserre company, one of the biggest property developers in Neuilly, France’s richest municipality.

As mayor, Mr Sarkozy granted permission for numerous Laserre projects, including his apartment building. According to Le Canard, the company also paid for lavish remodelling before the Sarkozys moved into the two-storey flat. They sold it last year fo 1.94 million euros.

In February, Mr Sarkozy called the allegations “hurtful and extravagant” and he produced receipts to show that he had paid for the installation of a new staircase. Le Canard reproduced other documents that appeared to confirm its claims, including evidence that smaller flats in the building had been sold for higher prices. But the affair faded when Ségolène Royal, Mr Sarkozy’s Socialist opponent, was accused by the media of evading wealth tax on her holiday home on the Riviera.

As President, Mr Sarkozy enjoys immunity from prosescution until he leaves office. His term ends in 2012 and he could seek re-election. Prosecutors are only now beginning to interview Jacques Chirac, his predecessor, over alleged corruption dating from his term as Mayor of Paries in the 1980s and early 1990s.

Mr Chirac suffered little public opprobium over far more spectacular allegations of sleaze than those involving Mr Sarkozy. However, public tolerance for the misdemeanours of French leaders is slowly waning. Suspicion that the President might have benefited from a shady deal will not help his honest-man image as he strives to persuade the French to give up welfare state privileges.

The prosecutor at Nanterre, a suburb neighbouring Neuilly, will have to decide whether to open a formal judicial investigation or to close the case. Le Canard predicted that it would be shelved. The prosecutor handling it is Philippe Courroye, a celebrated former investigating judge who is close to the President. Dominique de Villepin, the last Prime Minister, complained to judges about the ties between the two men last week

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