Europe’s top human rights court has ordered Russia to release jailed opposition leader Alexei Navalny a ruling quickly rejected by Russian authorities who are bent on isolating the Kremlin’s most prominent foe.

The ruling by the European Court of Human Rights (ECHR) that was posted on Mr Navalny’s website on Wednesday demands that Russia set him free immediately and warns that failing to do so would mark a breach of the European human rights convention.

Russia’s justice minister dismissed the court’s demand as “unfounded and unlawful”.

Mr Navalny, 44, an anti-corruption investigator and president Vladimir Putin’s most prominent critic, was arrested last month on his return from Germany, where he spent five months recovering from a nerve-agent poisoning that he blames on the Kremlin. Russian authorities have rejected the accusation.

Earlier this month, a Moscow court sentenced Mr Navalny to two years and eight months in prison for violating the terms of his probation while recuperating in Germany.

The sentence stems from a 2014 embezzlement conviction that Mr Navalny has rejected as fabricated and the European court has ruled to be unlawful.

In its ruling on Tuesday, the ECHR pointed to Rule 39 of its regulations and obliged the Russian government to release Mr Navalny, citing “the nature and extent of risk to the applicant’s life”.

“This measure shall apply with immediate effect,” the Strasbourg-based court said in a statement.

The court noted that Mr Navalny has contested the Russian authorities’ argument that they had taken sufficient measures to safeguard his life and well-being in custody following the nerve agent attack.

Russian justice minister Konstantin Chuichenko rejected the court’s ruling as a “clear and crude interference” in Russia’s judicial system.

“This demand is unfounded and unlawful because it doesn’t indicate a single fact or a legal norm that would allow the court to make such verdict,” Mr Chuichenko said in a statement carried by Russian news agencies.

“This demand can’t be fulfilled because there is no legal reason for that person to be released from custody under Russian law.

“Well aware of that, the European judges clearly have made a political decision that could only exacerbate restoring constructive relations with the Council of Europe’s institutions.”

In the past, Moscow has abided by the ECHR’s rulings awarding compensations to Russian citizens who have contested verdicts in Russian courts, but it has never faced a demand by the European court to set a convict free.

In a reflection of its simmering irritation with the European court’s verdicts, Russia last year adopted a constitutional amendment declaring the priority of national legislation over international law.

Russian authorities might now use that provision to reject the ECHR’s ruling.

Mikhail Yemelyanov, a deputy head of the legal affairs committee in the Kremlin-controlled lower house of parliament, pointed at the constitutional change, noting that it gives Russia the right to ignore the ECHR’s ruling, according to the Interfax news agency.

Mr Navalny’s arrest and imprisonment fuelled a wave of protests across Russia. Authorities responded with a sweeping crackdown, detaining about 11,000 people, many of whom were fined or given jail terms ranging from seven to 15 days.

Russia has rejected Western criticism of Mr Navalny’s arrest and the crackdown on demonstrations as meddling in its internal affairs.

A court hearing about Mr Navalny’s appeal of his sentence is scheduled for Saturday.

He has also faced court proceedings in a separate case on charges of defaming a Second World War veteran.

Mr Navalny, who called the 94-year-old veteran and other people featured in a pro-Kremlin video “corrupt stooges”, “people without conscience” and “traitors”, has rejected the slander charges and described them as part of official efforts to disparage him.