Rumours about Nicolas Sarkozy's marriage may have been motivated by a "financial" plot to destabilise him, according to his chief communication adviser.

Pierre Charon said Mr Sarkozy, the French president, could have fallen foul of a "sort of organized plot with financial movements", after reports surfaced online that both he and his wife Carla Bruni-Sarkozy were having affairs.

Mr Charon said this would be one among several lines police would be looking into following the launch of a preliminary criminal inquiry into the origin of the internet rumours.

Claims that Mr Sarkozy was having an affair with Chantal Jouanno, the ecology minister, and Mrs Bruni-Sarkozy with Benjamin Biolay, a singer, first circulated on Twitter in February.

France's first couple denied the rumours, which the president dismissed as idiotic "rantings".

They were relayed by at least two French journalists – without substantiation - on their Twitter accounts and in a blog on the website of the Journal du Dimanche, a respected Sunday newspaper whose owner, Arnaud Lagardère is said to be close to the president.

When Mr Sarkozy complained, the blogger – a 23-year-old brought in to "create a buzz" on the site - was sacked and Michael Amand, the director of the site, was forced to resign.

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