Russia sentenced a prominent critic of President Vladimir Putin to 25 years in prison in the harshest sentence yet handed down to an opposition activist.

Vladimir Kara-Murza, 41, a persistent campaigner against Putin’s rule who has condemned Russia’s war in Ukraine, was found guilty by a Moscow court of treason and other charges for criticizing the invasion, Russian news services reported. Kara-Murza had also actively lobbied for sanctioning Russian officials.

His sentence, to be served in a strict-regime penal colony, is by far the longest yet handed down in the Kremlin’s crackdown on opponents, which has sharply accelerated since the invasion in February 2022. His lawyer vowed to appeal, according to the state-run Tass news service.

Kara-Murza, one of a small number of activists who didn’t flee into exile after the start of the war in Ukraine, was arrested in April 2022 on suspicion of spreading false information about the armed forces and declared a “foreign agent.” Prosecutors later charged him with treason over a speech made before the Arizona House of Representatives denouncing the Russian attack on Ukraine.

The UK condemned the “politically-motivated” conviction of Kara-Murza, who has both Russian and British citizenship, echoing US and European Union criticism of the case.

In a defiant appearance at the closing of his trial last week, Kara-Murza compared the proceedings to the show trials in the 1930s under Soviet dictator Josef Stalin.

“I’ve been surprised by the extent to which my trial, in its secrecy and its contempt for legal norms, has surpassed even the ‘trials’ of Soviet dissidents in the 1960s and ’70s. In this respect, we’ve gone beyond the 1970s — all the way back to the 1930s,” said Kara-Murza, who has accused the authorities of twice poisoning him in the past.

“I’m in jail for my political views. For speaking out against the war in Ukraine. For many years of struggle against Vladimir Putin’s dictatorship. Not only do I not repent for any of this, I am proud of it.”

His treason conviction comes as Russia has expanded the legal definition of that crime to cover disclosures of public information that the Kremlin deems harmful to its interests.

“This is a warning to all anti-Putin activists - don’t come back or we’ll lock you up, de facto for life,” Tatiana Stanovaya, founder of the R.Politik political consultancy, wrote in Telegram.

The activist’s sentence dwarfs the 8 1/2-year jail term given in December to another anti-government campaigner, Ilya Yashin, an ally of imprisoned opposition leader Alexey Navalny. Mikhail Khodorkovsky, once Russia’s richest man, was sentenced to 14 years for fraud in what he said was revenge for his political activities, and freed under amnesty in 2013 after more than a decade in prison.

“This is the harshest ever punishment for an opponent of Putin,” said Ivan Pavlov, a human rights lawyer who specialized in defending people accused of treason and fled Russia in 2021.

Navalny, 46, who’s serving two sentences totaling 11 1/2 years, faces a new trial soon on charges including promoting extremism that carry a maximum 35-year sentence, according to his supporters.

He’s suffering from an unidentified stomach ailment that may be an attempt to poison him slowly, his spokeswoman Kira Yarmysh said April 12.

After losing 8 kilograms (18 pounds) over 15 days of solitary confinement, an ambulance was called for Navalny because of acute stomach pain, she said. Three days after being released from the punishment cell, he was put into it for another 15 days on April 10 and is being held there “with acute pain without medical help,” she said on Twitter.