Muammar Gaddafi's son has claimed that Libya helped finance Nicolas Sarkozy's successful election campaign in 2007, and demanded that the French president return the money to "the Libyan people".

In an interview with the Euronews TV channel, Saif al-Islam Gaddafi said Libya had details of bank transfers and was ready to make them public in a move designed to punish Sarkozy for throwing his weight behind opposition forces.

Last week, the Libyan government threatened to reveal a "grave secret" that would bring down Sarkozy, with Saif al-Islam calling him "a clown".

The regime is furious at Sarkozy's efforts to galvanise international action to impose a "no-fly zone" that would prevent Gaddafi from using air power against rebels based in Benghazi.

Asked what he felt about the French president's so far unsuccessful efforts to muster support for military intervention, Saif said: "Sarkozy must first give back the money he took from Libya to finance his electoral campaign. We funded it. We have all the details and are ready to reveal everything. The first thing we want this clown to do is to give the money back to the Libyan people. He was given the assistance so he could help them, but he has disappointed us. Give us back our money."

Libya has yet to release any incriminating evidence but officials hinted last night that they were preparing to do so.

A spokeswoman for the Elysée Palace told the Guardian she had no information or comment about the claim. But Le Monde later quoted a spokesman as saying: "We deny it, quite evidently."

Libyan sources have separately told the Guardian substantial funds were paid into accounts to support Sarkozy's presidential campaign in 2007.

Well-placed sources in Tripoli made clear that the leak of this information was in retaliation for France's leading role in the campaign to impose a no-fly zone and for its unique recognition of the rebel Libyan National Council. "Sarkozy is playing dirty, so we are playing dirty, too," said a senior Libyan source.

The Guardian has been unable to confirm the Libyan claims independently.

French law places strict limits on party donations to candidates. Last year, Sarkozy was hit by a political scandal involving alleged illegal donations to his party funds by France's richest woman, Liliane Bettencourt.

Eyebrows were raised when Gaddafi visited Paris in late 2007 and was permitted to pitch his trademark bedouin tent in the gardens of the Hotel Marigny, the 19th-century mansion close to the Elysée Palace, which hosts visiting VIPs. That triggered a storm of adverse comment about the warmth of his reception by Sarkozy on international human rights day.