Western governments and Russia’s exiled opposition should begin laying the groundwork for Russia’s democratic transition after President Vladimir Putin eventually leaves office, Vladimir Kara-Murza, a Russian opposition politician, has said.

Seven weeks after he was released from a Siberian penal colony in a historic East-West exchange, Kara-Murza did not say how he thought Putin would leave, but said on Friday that Russia must not squander what he said would be a narrow sliver of time to establish a democratic government, as he said it did after the Soviet Union’s collapse in 1991.

“We need to learn from those past mistakes, from those past lessons, to make sure we do not repeat these failures the next time a window of opportunity for change in Russia opens,” Kara-Murza told reporters at the Royal United Services Institute, a London think tank, in his first public appearance in the United Kingdom since he was released on August 1.

“None of us knows exactly when, exactly in what circumstances, but it’s going to happen in the very foreseeable future. And next time, we must get this right.”

Putin, 71, has been in office as president or prime minister since 1999. He began a new six-year term as president in May and dominates the political landscape in Russia, with leading opposition figures in prison or in exile.

Kara-Murza, 43, has emerged as one of the most prominent opposition voices in exile since his release from prison, where he was serving a 25-year treason sentence for publicly opposing the war in Ukraine. He holds Russian and British passports.

“Vladimir Putin must not be allowed to win this war in Ukraine. More than that, he must not be allowed to have a face-saving exit from this war,” he said on Friday.

He argued the West should be preparing a plan for a future democratic Russia, which should include Western leaders communicating to the Russian people that the West stands with them against Putin, Kara-Murza said.

Securing the release of more prisoners of conscience – who he said number about 1,300 in Russia – is key.

“I wake up every morning and I go to sleep every night thinking about all the others who are still left behind,” the politician said.

He highlighted 63-year-old Alexei Gorinov, the first person jailed under Russia’s wartime censorship laws, and Maria Ponomarenko, a Siberian journalist currently on hunger strike in prison, as among those in dire need of support.

Asked whether he was concerned prison swaps could encourage Putin to take more captives, Kara-Murza said he would continue to take prisoners in any case “because he is afraid of the truth”.

Arguing that the prisoner swap on August 1 had saved “16 human souls” from the “hell” of Russia’s prisons, he added, “It wasn’t a prisoner exchange, it was a life-saving operation and we need to look at it this way.”