A Moscow court ruled Tuesday that investigators acted lawfully when they refused to look into two alleged murder attempts against the jailed Kremlin critic Vladimir Kara-Murza.

Kara-Murza, 42, fell severely ill in 2015 and 2017 with symptoms that he blamed on poisoning, but Russia's Investigative Committee did not open an inquiry after the allegation.

An investigation led by Bellingcat journalists in 2021 suggested that FSB agents were involved in both cases.

"The investigators, having received this information, did nothing," Kara-Murza told the court via video link from Siberia, where he is serving a 25-year prison sentence for treason and other charges.

"As a result of the poisonings I ended up in intensive care with a five percent chance of survival," he said.

Fears for the prominent dissident's health have grown since fellow opposition figure Alexei Navalny died in prison under opaque circumstances in February.

Kara-Murza, a citizen of both Russia and Britain, suffers from serious health problems that his wife Evgenia and lawyers say are directly related to the alleged poisoning attempts.

He was imprisoned in April 2023 after criticising Russia's military assault on Ukraine and pressing Western countries to impose sanctions against the Kremlin.

Russia's top court upheld the 25-year sentence against him on May 14, throwing out an appeal from his lawyers, who called his jailing unlawful.

Since launching its offensive in Ukraine, Russia has detained, jailed or exiled thousands of dissidents in a campaign critics say carries echoes of the mass repression seen during the Soviet Union.