Vladimir Kara-Murza, 41, a father of three and former journalist who holds Russian and British passports, spent years as a politician opposing the Russian president.

He was accused of treason and discrediting the military by prosecutors after he criticised the country's invasion of Ukraine.

His 25-year sentence, which matches the sentence requested by prosecutors, is the harshest of its kind since Russia's invasion.

Mr Kara-Murza, who stood by his words throughout his trial, previously criticised his prosecution and compared it to one of Soviet dictator Josef Stalin's show trials in the 1930s.

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He declined to ask the court to acquit him and said: "Criminals are supposed to repent of what they have done. I,
on the other hand, am in prison for my political views."

"I also know that the day will come when the darkness over our country will dissipate," he said during his trial.

'Politically motivated'

Mr Kara-Murza's sentencing attracted criticism from the British government, which described the conviction as "politically-motivated".

The Foreign, Commonwealth and Development Office (FCDO) said it had summoned the Russian Ambassador, Andrey Kelin, and that it will "make clear that the UK considers Mr Kara-Murza's conviction to be contrary to Russia's international obligations on human rights, including the right to a fair trial".

Foreign Secretary James Cleverly said: "Vladimir Kara-Murza bravely denounced Russia's invasion of Ukraine for what it was - a blatant violation of international law and the UN Charter.

"Russia's lack of commitment to protecting fundamental human rights, including freedom of expression, is alarming.

"We continue to urge Russia to adhere to its international obligations including Vladimir Kara-Murza's entitlement to proper healthcare.

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"I pay tribute to Mr Kara-Murza and his family, including his wife Evgenia and her unwavering commitment to raising her husband's cause on the international stage.

"The UK will continue to support them and to call for Mr Kara-Murza's immediate release."

'Regime of murderers'

Under censorship laws, introduced in the wake of Russia's invasion of Ukraine, "discrediting" the army can currently be punished by up to five years in prison.

Spreading information deemed deliberately false about the Russian military can attract a 15-year jail sentence.

Mr Kara-Murza had used speeches in the United States and across Europe to accuse Moscow of bombing civilian targets in Ukraine - which the Kremlin has denied.

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He also claimed Russia was being run by a "regime of murderers" in a CNN interview, broadcast hours before his arrest.

The charges against Mr Kara-Murza relate to a speech given to the Arizona House of Representatives in March, in which he denounced Russia's military action in Ukraine.

He claimed soldiers were "dropping cluster bombs on residential areas, mothers' homes, hospitals, and schools".

Mr Kara-Murza was declared a "foreign agent" and accused of spreading false information in the aftermath of the speech.

Investigators added a treason charge, which carries a sentence of up to 20 years in Russia, while he was in custody over public speeches he had made in Lisbon, Helsinki and Washington.

Mr Kara-Murza, who was once deputy leader of the opposition group People's Freedom Party, is the current vice chairman of the pro-democracy and human rights group Open Russia.

He was an ally of the late Russian opposition politician Boris Nemtsov, who was shot dead near the Kremlin in 2015.

Mr Kara-Murza and his supporters say he has twice survived being poisoned in the past - once in 2015 and again in 2017 - though Russian authorities have denied any involvement.