A London-based anti-Putin campaigner has called on Boris Johnson to freeze the assets of thousands of Russian citizens in the UK in a bid to root out those with ties to the Kremlin.

Andrei Sidelnikov said he told the prime minister to impound the property and bank accounts of emigrants with unexplained wealth, connections with the Russian embassy in London, ties to pro-Putin oligarchs, and of those involved in organising cultural events linked to the Russian state.

Sidelnikov, a former associate of the former KGB agent Alexander Litvinenko who was murdered in London in 2006, said their assets should be confiscated while corruption investigations were conducted, adding that he was compiling a list of suspected Putin supporters that would be shared with the UK government.

The opposition activist said his campaign group, Speak Up!, had founded a new project called Revenge, aimed at gathering the names of Russian emigrants who support Vladimir Putin.

“From this moment, all the people who supported this criminal regime must answer for this,” said Sidelnikov, who received political asylum in Britain in 2008 after telling officials his life would be in danger if he remained in Russia. “[If we] don’t close such doors for supporters of criminal Putin’s regime then the war will come to our homes.”

In a letter sent to Johnson after Russia invaded Ukraine last month, Sidelnikov also called on the UK government to cancel diplomatic relations with Russia, stop granting visas to Russian citizens until stricter immigration checks were put in place, and to push Nato to impose a no-fly zone over Ukraine.

He added that British lawyers and financiers who helped corrupt Russian emigrants should face tougher legal penalties. “In the future, it must be a criminal offence to have relations with Russians who are supporting the fascist regime.”

Although the campaigner admitted there was a danger that such action could fuel Russophobia in the UK, he added: “The problem is that there are so [many] Russian citizens who supported Putin’s criminal regime. It’s very difficult to find them because it’s thousands, not 10 or 100, or 700.

“The Russian community is very angry about what I’m saying because so many of them have some links with Putin’s regime.”

Sidelnikov said that before the invasion of Ukraine he had sent the prime minister a list of 99 Russian citizens living in the UK who supported Putin.

Since the invasion, he said, more than 100 names had been added to this database, adding that his organisation would continue to gather information on allegedly corrupt Russian emigrants for the duration of the war.

“The plan is to give this not only to UK authorities, but to give this to the authorities in other countries, including the European Union and the US,” he said.

The activist said the names included people who had not been publicly identified as Putin supporters.

“We know those people who are living here and there are no sanctions on them,” he added. “They are trying to corrupt British society.”

He said a task force, consisting of the police, security services, government officials and MPs, should be established to investigate those accused of corruption.

Sidelnikov, 46, added that some emigrants in the UK were now pretending that they had never supported the Russian president.

“Some people are turning around after all the years they supported his regime, and now they’re putting on their Facebook, oh, we don’t want war, we’re not for war.”

The campaigner predicted that Russia would lose the war with Ukraine. But he expected that Putin would retain the presidency for the foreseeable future.

“There is no opposition,” Sidelnikov said. “Some of them were killed like my friend Litvinenko. A lot of my friends are in prison now. Some of them escaped from Russia.”