Former French president Jacques Chirac has refused to testify to an inquiry into the so-called Clearstream affair.
Jacques Chirac says he is covered by presidential immunity
Judges want to know if Mr Chirac ordered a secret probe into Nicolas Sarkozy over corruption claims, before he succeeded him as president.
Allegations that Mr Sarkozy received illicit funds have been disproved.
Mr Chirac has agreed to answer questions over a separate alleged financial scandal during his time as Paris mayor, a French newspaper said.
In that investigation, a judge has asked to question the former president over investigations into a party financing affair linked to his time as mayor from 1977 to 1995, Le Parisien newspaper said.
French magistrate Alain Philibeaux is looking into allegations of fake jobs at Paris city hall, and the payment of millions of dollars of salaries and fees to members of Mr Chirac's former party, Rally for the Republic.
Mr Chirac has denied any wrongdoing.
Mr Chirac said he would not testify over the Clearstream case because he was protected by his presidential immunity.
His office said that under the constitution, he could not be made to testify on things that were "done or known during his mandate and in carrying out his functions".
The statement added that the former president was ready to answer any questions about cases that predated his term in office.
The Clearstream affair, which transfixed France a year ago, centres on claims that a number of prominent political and business figures benefited from illegal commissions paid as part of a big arms sales contract to Taiwan.
The claims have since been shown to be completely false.
But the BBC's Hugh Schofield in Paris says the interest lies in the fact that among the names on the faked account list at the Clearstream Bank of Luxembourg was that of France's president - then interior minister - Nicolas Sarkozy.
The allegation concerning Mr Chirac is not that he was in any way involved with the scam but that having learned of it, he may have set up an intelligence inquiry to see if the claims against Mr Sarkozy were true.
At the time relations between the two men were tense and Mr Chirac was not believed to be in favour of Mr Sarkozy's presidential candidacy.
The allegation is that he was fishing to see if there was something that might be held against Mr Sarkozy, according to our correspondent.