A downgrade of France's cherished triple-A credit rating would be painful and costly and could take years to repair, the head of the country's debt management agency said in an interview with daily Les Echos.

Agence France Tresor (AFT) Chief Executive Philippe Mills was quoted as saying a downgrade for France would be a blow to the entire euro zone.

Moody's rating agency warned on Monday that a sustained rise in France's debt yields coupled with weakening economic growth could harm the country's ratings outlook, fuelling concern the euro zone's No. 2 economy risks losing its AAA status.

I can't say what the exact cost of financing for France would be, given market volatility, but those who think nothing would happen if France lost its AAA rating are completely mistaken, Mills said.

A financing surcharge of 120 basis points represents around 2.5 billion euros (2.1 billion pounds) extra per year. We should also bear in mind that getting back to a AAA rating takes time, sometimes a decade, he said, taking the example of AA+ rated Belgium which currently pays around 120 points more than France on its debt.

A downgrade would have consequences on the rest of the euro zone and on all French issuers, even private companies, he said.

Worries about a high deficit and French banks' exposure to troubled euro zone peripherals have drawn France into the firing line of the bloc's escalating crisis, despite fresh efforts by the centre-right government to shore up public finances.

Moody's said in mid-October that it could place France on negative outlook in three months if the cost of helping to bail out banks and other euro zone members overstretched its budget.

Mills said the AFT would decide on Friday whether to go ahead with an optional debt issue in December, and said it was possible the agency would reduce outstanding short-term debt more than first expected by year-end.

(Reporting By Catherine Bremer; Editing by John Stonestreet)