A criminal inquiry is under way in France to find the origin of internet rumours that President Sarkozy and Carla Bruni-Sarkozy, his wife, were having affairs.

The investigation was made public because the President’s advisers suggested that the rumours might have been started in an attempt to destabilise Mr Sarkozy’s position at a time when he is seeking to regulate global capitalism.

“Now we are going to see whether there is a sort of organised plot with financial movements,” said Pierre Charon, Mr Sarkozy’s communication adviser. “There had to be a judicial procedure, so that fear would switch to the other side.”

Claims that Mr Sarkozy was having an affair with Chantal Jouanno, the Ecology Minister, and Ms Bruni with Benjamin Biolay, a singer, circulated on Twitter in February.

Mr Sarkozy and his wife have denied the rumours, which the President dismissed as “idiotic”.They were repeated by at least two French journalists on their Twitter accounts — although both say that they have no evidence to substantiate the allegations — and in a blog on the website of the Journal du Dimanche, a newspaper whose political coverage is regarded as largely non-partisan.

When Mr Sarkozy complained, the blogger was sacked and Michael Amand, the director of the site, was forced to resign.

Under pressure from Mr Sarkozy’s lawyers, Hachette Filipacchi Associés, which owns the Journal du Dimanche, has lodged a formal legal complaint for “fraudulent introduction of data into a computer system”.