PARIS (Reuters) - Election auditors believe that French ex-president Nicolas Sarkozy overshot the legal spending limit during a failed bid to secure re-election, his campaign treasurer said on Friday, vowing to contest the estimate.

Sarkozy, who has already faced questions over funding of his successful 2007 campaign, declared that spending on this year's bid for a second term fell slightly short of the legal limit, which is 22.509 million euros ($29.80 million).

Beyond any blemish for the former leader, who some in his camp would like to see run again in 2017, Sarkozy's conservative UMP party could lose out on millions of euros in state funding if financial penalties are imposed.

The national campaign financing watchdog, the CNCCFP, judged that meetings Sarkozy held before he officially declared himself in the running incurred costs that pushed him over the limit, according to Sarkozy's campaign treasurer Philippe Briand.

"I formally contest the CNCCFP's analysis and decisions," Briand said in a statement.

He said he would appeal the "unprecedented decision" in the Constitutional Council, which has the last word on approval of election finances.

In France, election funding is capped, large donations are banned and the state refunds spending.

Sarkozy, who formally declared himself a re-election candidate in February, reported total spending of 21,339,664 euros, short of the 22,509,000 euro limit.

The CNCCFP declined to comment pending discussion of the matter at the Constitutional Council.

News magazine L'Express said an overspend could potentially cost the UMP 11 million euros in forfeited state subsidies.

Sarkozy, who has stepped back from frontline politics since his May 6 defeat by Socialist Francois Hollande, has also come under scrutiny over funding of his 2007 election.

He has strenuously denied any wrongdoing and repeated that when questioned in November by investigators who are trying to establish whether there is anything to allegations that he may have got illicit funds back then from France's richest woman, L'Oreal heiress Liliane Bettencourt.

(Reporting by Thierry Leveque, Chine Labbe and Leigh Thomas; Editing by Brian Love and Catherine Evans)