Russian agents were assigned to assassinate former KGB agent Alexander Litvinenko, a European court ruled, while U.K. prosecutors charged a third Russian in relation to a 2018 chemical attack in England, thrusting the Kremlin’s alleged role in poisoning its opponents back into the spotlight.

Mr. Litvinenko a prominent critic of President Vladimir Putin, died in 2006 after drinking tea that was laced with polonium-210, a deadly radioactive isotope, at a bar in London.

The...

Russian agents were assigned to assassinate former KGB agent Alexander Litvinenko, a European court ruled, while U.K. prosecutors charged a third Russian in relation to a 2018 chemical attack in England, thrusting the Kremlin’s alleged role in poisoning its opponents back into the spotlight.

Mr. Litvinenko a prominent critic of President Vladimir Putin, died in 2006 after drinking tea that was laced with polonium-210, a deadly radioactive isotope, at a bar in London.

The European Court of Human Rights on Tuesday said Andrei Lugovoi, a former KGB bodyguard, and Dmitry Kovtun, a onetime Soviet army officer, had poisoned Mr. Litvinenko. The court ruled they “had been acting as agents of the Russian State.”

“Mr. Litvinenko’s assassination was imputable to Russia,” the court said, and the “Russian Government had failed to provide any other satisfactory and convincing explanation of the events.”

Dmitry Kovtun, left, and Andrei Lugovoi have denied any involvement in the death of Mr. Litvinenko.

Photo: Sergei Karpukhin/REUTERS

U.K. prosecutors and a British public inquiry have concluded that Mr. Lugovoi and Mr. Kovtun were behind the poisoning. Both men have previously denied any involvement in Mr. Litvinenko’s death. Mr. Lugovoi, who is currently a deputy in Russia’s State Duma, said Tuesday that the court’s judgment was “wrongful, unlawful, and politically biased,” according to Russian news agency Interfax.

Russia, like the U.K., is a member of the Council of Europe, which oversees the court.

The ruling came the same day that U.K. prosecutors charged a third Russian agent in relation to the 2018 chemical attack in Southern England that targeted former Soviet spy Sergei Skripal and his daughter Yulia.

U.K. officials allege that Russian operative Denis Sergeev was involved in the poisoning plot that hospitalized the Skripals and killed another person in the city of Salisbury. British authorities have already charged two men who they allege were Russian military intelligence officers.

The Kremlin has repeatedly denied involvement in the Salisbury poisoning and rejected accusations of involvement in Mr. Litvinenko’s death. Presidential spokesman Dmitry Peskov called the court’s ruling in Mr. Litvinenko’s case baseless.

“It’s unlikely that the [European Court of Human Rights] has the authority or the technological capabilities to have information on this matter,” Mr. Peskov told reporters during a conference call.

He didn’t immediately comment on the charges brought against the third alleged Russian agent in the Skripal case.

Elena Panina, a member of the State Duma’s Committee on International Affairs, described the charging of Mr. Sergeev as “an episode of hybrid war against Russia,” according to the Russian state news agency RIA Novosti. “Today we see a new version of the old Russophobia,” she said.

The Russian Embassy in London said Moscow had initially offered full and open cooperation between the law enforcement agencies of both countries to establish the facts of what happened in Salisbury. But it said “the British preferred to conduct the investigation completely in secret, behind closed doors,” adding that no specific facts had been presented to confirm Mr. Sergeev’s alleged involvement in the poisoning of the Skripals.

The embassy said Russia was still open “for professional cooperation” so long as the U.K. “abandons the concept of a politically motivated appointing of the guilty.”

According to information published by the court, Mr. Litvinenko had worked for Soviet security services the KGB and its successor the Federal Security Service, or FSB. In 1998 he alleged that he had been asked to examine the possibility of assassinating a wealthy businessman, the court said. He was fired from the security service and fled Russia for the U.K., where he and his family were granted asylum and later citizenship.

While in Britain, Mr. Litvinenko wrote a book accusing Mr. Putin of coming to power by organizing a series of apartment bombings in 1999 that prompted Russia’s invasion of Chechnya. The Kremlin has denied the charges.

—Max Colchester in London contributed to this article.

Write to Ann M. Simmons at ann.simmons@wsj.com