Alexei Navalny, the jailed critic of Vladimir Putin, is grappling with a mystery ailment in prison that could be the result of a slow-acting poison, his spokesperson has suggested.

The opposition politician has reportedly lost 8kg in weight in just over two weeks and an ambulance was called for him over the weekend.

Mr Navalny is being held in a maximum-security IK-6 penal colony at Melekhovo, about 115 miles east of Moscow.

His spokesperson Kira Yarmysh says an unknown stomach complaint flared up and prison doctors had treated him in the past by injecting him with medicine which they had refused to identify.

“We do not rule out that at this very time Alexei Navalny is being slowly poisoned, being killed slowly so that it attracts less attention,” she said in the Twitter post.

“He is being held in a punishment cell with acute pain without medical help,” she added.

When asked about claims that Mr Navalny might be being slowly poisoned, the Kremlin said it was not following the state of his health and that it was a matter for the federal prisons service.

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Mr Navalny has long been an outspoken critic of Mr Putin.

In March, he was sentenced to nine years in prison after he was found guilty of large-scale fraud and contempt of court.

He has called the Russian invasion of Ukraine “stupid” and “built on lies”, while calling on fellow citizens to stage daily protests.

Mr Navalny was arrested in January 2021 after he returned to Russia from Germany, where he had been recovering from nerve agent poisoning.

He had fallen ill on a flight to Moscow and was subsequently found to have been poisoned with novichok during a campaign trip to Siberia.

A documentary about Mr Navalny won an Oscar earlier this year.