“Navalny’s situation is extremely concerning,” Le Drian told France 3 television. “I hope that measures are taken to ensure Navalny’s physical integrity, but also his freedom,” he said.

“There is a major responsibility here for President (Vladimir) Putin.”

Navalny, Putin’s most prominent opponent, was arrested in January on returning to Russia after recovering from a near-fatal poisoning attack he says was orchestrated by the Kremlin. The Kremlin denies the allegation.

He is serving two and a half years on old embezzlement charges – which he says are politically motivated – in a penal colony in the town of Pokrov around 100km east of Moscow.

The 44-year-old began a hunger strike on March 31 to demand proper medical treatment for back pain and numbness in his legs and hands.

On Saturday, several doctors close to Navalny warned that his health had rapidly deteriorated and he could “die any minute”, demanding prison officials grant them immediate access.

“There is truly an authoritarian drift in Russia,” Le Drian said.

“Russia is responsible for Navalny’s health, it must assume its role,” he added, saying the EU was closely following his fate.

“We have already taken action,” he said, referring to EU sanctions.

“The package of sanctions is already significant, but there may be others.”