The message on Twitter by Aude Baron, a French online journalist, and repeated by other reporters, set the Internet alight.
And soon, the rumour spread like wild fire with gossip columnists and newspapers around the world chasing the story, and eventually forcing Sarkozy into an angry denial during a press conference with Gordon Brown in London last week.
However, it has now been revealed that the reporters who spread the claims said that they did not have the remotest idea whether they were true.
Even Baron said that she had merely repeated 'gossip' without checking it.
"It was a kind of joke," the Times quoted her as saying.
It was rumoured that Bruni is having a liaison with Benjamin Biolay, the singer, and her husband, Nicolas, with Chantal Jouanno, the Ecology Minister. The ensuing row has sparked a debate in France over the legal status of Twitter.
And some users have claimed that it is a private forum where you can say what you want with no risk of being sued.
Mark Stephens, a leading London libel lawyer, said that they were wrong.
"Libel, privacy, obscenity, harassment and other laws apply with equal force to all forms of media," he said.