The two now-retired DGSE (General Directorate for External Security) agents, and the wife of one of them, are being prosecuted for "acts of extreme gravity", the armed forces ministry said in a statement.

They were indicted on 22 December 2017 and two of them have since been held in custody, a judicial source told AFP.

DGSE is France's foreign intelligence agency, similar to Britain's MI6 or the United States's CIA.

The Quotidien television programme and the Le Monde daily reported four people are suspected of having been recruited by the Chinese authorities to spy on French foreign intelligence.

Neither the armed forces ministry nor the judicial source gave the name of the other country, and gave no details of the length of time or nature of information passed over.

One of the agents was stationed in Beijing, Quotidien reported.

The judicial source said two of the three suspects are being prosecuted for "delivering to a foreign power information that undermines the fundamental interests of the nation" and "compromising the secrecy of national defence".

"One of them has also been charged for direct provocation to the crime of treason," the source added.

The third person -- believed to be the wife -- has been indicted for "concealment of treasonable crimes" and placed under judicial control, meaning they are subject to certain constraints pending trial, according to the same source.

The armed forces ministry said: "These acts of extreme gravity have been detected by this service, which has brought these facts to its knowledge to the Paris prosecutor."