Both were placed under formal investigation on December 22 to face charges of spying for a foreign power, compromising classified secrets and delivering information detrimental to fundamental national interests. One of the former agents, whose identities have not be disclosed, also faces charges of directly inciting treason, the source said.

The third person - believed to be the wife of one of them - has been indicted for "concealment of treasonable crimes".

According to Le Monde, the two ex-spies are suspected of handing over information on "the working methods" of France's external intelligence service.

"The fact that we sounded the alert is proof of our vigilance," Parly said. "France has partners, but we live in a dangerous world, and unfortunately these types of things can happen."

The rare public acknowledgement of the arrest of suspected double agents came as Australia's spy chief Duncan Lewis told a Senate estimates hearing that foreign [read Chinese] espionage was being conducted in Australia at an unprecedented scale that could cause "catastrophic harm".

Lewis, head of the Australian Security Intelligence Organisation (ASIO), did not single out a country, but his remarks coincided with a sharp escalation of concerns over Chinese interference in domestic politics.

Telegraph, London