A mysterious Russian billionaire has trumped his big-spending rivals and broken a world record by splashing out $750 million on one of the most sumptuous villas on the French Riviera.

The price of the Villa Leopolda, a Belle Apoque mansion on the heights of Villefrance, has amazed estate agents but fuelled local worries that the invasion of Russian money on the Cote d'Azur is getting out of hand.

Since the early 1990s, Russian oligarchs, drawn by memories of the Riviera-mad old Russian aristocracy, have been piling into seaside properties at Cap Ferrat, Cap d'Antibes, Saint-Tropez and the other great playgrounds.

None, however, has come near the price with which the unnamed Russian clinched the Leopolda deal with Lily Safra, the widow of Edmond Safra, a Lebanese banker who was killed by an arsonist's fire in Switzerland in 2003.

Lily Safra was said to have held out for months as the buyer raised his bid for the villa, between Nice and Monaco, which King Leopold II of Belgium acquired in 1902.

The previous record for a house was said to be the $110 million that Lakshmi Mittal, the steel tycoon, paid for a property in Kensington Palace Gardens in 2004. The macho spending contest by Russian oligarchs is making property agents giddy.

Jean Pierre, a high-end agent, said: "It's completely surreal and we are really uneasy. We don't dare any more to propose any price below $150 million for these clients. Anything below and they throw you out...and you should see how they do it," he told Le Parisien. The Nice Matin newspaper said: "At this price tag, we are beyond luxury and even reality."

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