PARIS (Reuters) - French presidential candidate Francois Fillon got the backing of prominent French businessman and HSBC board member Henri de Castries on Tuesday, with his long-term friend confirming a move into politics.

De Castries, who left AXA in September after nearly 17 years leading Europe’s second-biggest insurer, had been regarded as frontrunner for taking over as chairman of HSBC after joining its board in March.

However, the 62-year-old started appearing at campaign events for Fillon, who in November became the conservative candidate in the presidential election and is the frontrunner in opinion polls to succeed Socialist President Francois Hollande.

It is not very common for corporate leaders to publicly back French presidential candidates and it is even more rare for them to get involved in campaigning or join government.

“Francois Fillon has the best answers for the challenges we face: security, solidarity, competitiveness, education and innovation,” de Castries told Le Figaro newspaper.

Fillon has said he wants non-professional politicians to join his government if elected in May. French media speculated at the end of last year, when he was advising Fillon unofficially, that de Castries could become finance minister.

De Castries, a pro-European, declined to say what form his support would take or if he wanted to become a minister.

An aristocrat known for his economically liberal ideas, de Castries has been criticized by union leaders and opposition politicians who saw him behind Fillon’s plans to radically overhaul France’s social security - something he denies.

Fillon has since toned these down after raising concerns even within his center-right The Republicans party.

De Castries, whose full name is de la Croix de Castries, is the descendant of an ancient aristocratic French family.

The family has counted among its ranks a defense minister and several centuries previously one of the few holders of France’s top military distinction.

A graduate of France’s elite ENA administrative school and HEC business school, de Castries was a civil servant before moving to Axa in 1989.