'I'm buying you J-Lo's greatest hits - and paying her £1m to sing them'

Andrei Melnichenko, 35, is paying £600,000 for the performance plus a further £400,000 to fly the singer's entourage in from the U.S. for the party in Berkshire and put them up in top London hotels.

Miss Lopez - who happens to bear a striking resemblance to Mr Melnichenko's wife Aleksandra - will sing just a handful of numbers in a show lasting less than 40 minutes.

She will perform hits such as If You Had My Love for Melnichenko's compatriot and fellow billionaire, Chelsea owner Roman Abramovich.

The night will be a joint party to celebrate Andrei's recent birthday and the 30th birthday of his wife - a former pop star and Miss Yugoslavia - at their large estate in Ascot.

Saturday's all-day bash will also include a circus act and cabaret.

Despite rumours of there being a long list of celebrities attending, the guests will include mainly Russian and Serbian tycoons.

Mr Melnichenko's spokesman in London would only say: "This is strictly a private party for close friends and family."

But a source close to Miss Lopez said: "It will be wall-to-wall billionaires in there. Andrei and Aleksandra are both very great fans of J-Lo, so Andrei put the call in and personally put the offer to her. She accepted immediately.

"She will sing for about 40 minutes - not bad money for the work."

As founder and owner of Moscow's MDM bank, and also a minerals magnate, babyfaced Mr Melnichenko is one of the world's youngest billionaires.

It will be a fitting celebration for the couple, who hired Christina Aguilera to sing at their 2005 wedding in the South of France for £2million.

The Daily Mail reported in January how George Michael pocketed £1.7million for performing at a Russian billionaire's New Year's Eve bash in Moscow.

He played an hour-long concert for Vladimir Potanin's 250 guests, becoming the highest-paid entertainer in modern Russian history.