Marina Litvinenko, whose husband was poisoned with radioactive polonium in 2006, told Sky News “nothing was done” following a public inquiry into Mr Litvinenko’s death.
She suggested Theresa May, home secretary at the time of the inquiry, has failed to act over her husband’s murder.
Mr Litvinenko was a former Russian FSB agent who went on to work with MI6 after fleeing to the UK.
His murder is suspected to have been personally signed off by Russia’s President Putin. Russia has denied any involvement.
Mrs Litvinenko said she was shocked at the recent poisoning of former spy Sergei Skripal and his daughter Yulia in Salisbury.
The nerve agent attack has cast fresh suspicions on Moscow’s activities in Britain.
Mrs Litvinenko revealed Mrs May wrote to her after the inquiry into her husband’s murder concluded in 2016.
In the letter, she vows to “take every step to protect the UK and its people from such a crime ever being repeated”.
Speaking to Sky News on Sunday, she said: “We received very strong words after meeting in 2016 and I believed something would be done, but after three years we can see nothing was done.”
Mr Skripal, who had acted as a double agent for the UK’s intelligence services, came to Britain after being released from a Russian prison under a spy swap in 2010.
Asked whether the attack on Mr Skripal demonstrates Britain is unable to protect those offered political asylum, Mrs Litvinenko said: “It looks like, after what happened to my husband, and you accepted people who might be targeted from a foreign country, it looks like you’re not able, for now.”
Mrs Litvinenko added her voice to those demanding a review of 14 other deaths on UK soil that have reportedly been identified by US intelligence sources as potentially connected to the Russian state.
“These cases were very quickly closed and I don’t think proper investigation was provided,” she said.
“I’m absolutely sure it must be done.”
Mrs Litvinenko stressed Russia can’t be blamed “immediately” for Mr Skripal and his daughter’s poisoning, but highlighted how those currently ruling in Moscow play by “different rules”.
Russia has dismissed any claims it might be behind the poisoning as mere “propaganda”.
Mrs Litvinenko also called on the Conservative Party to return any donations from Russian-linked individuals or businesses if they cannot determine the origin of the cash.
It comes after The Sunday Times revealed Russian oligarchs and their associates have registered donations of more than £820,000 to the Tories since Mrs May became Prime Minister.
“Not every Russian person is a criminal, even if the person is rich,” Mrs Litvinenko said.
“But you need to be very accurate where this money came from before you accept this money.”
She added: “If you identify it’s dirty money [you’re] just not allowed to accept it.
“Because I think reputation is very important. [The] reputation of the Conservative Party in UK and all around the world need to be clear.”
A spokesman said: “All donations to the Conservative Party are properly and transparently declared to the Electoral Commission, published by them, and comply fully with the law.”
Responding to Mrs Litvinenko’s claim that the Government had failed to act adequately against Russia after her husband’s murder, Chancellor Philip Hammond told the BBC’s Andrew Marr show: “The inquiry of course took some time. And it was some time after the events that we had the evidence from the inquiry.
“But we took the appropriate steps, we took measures which are still in place today.
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“But, of course, the Russians have not complied with their international obligations despite being members of the [United Nations’] Security Council.
“They have continued to protect those who we seek to extradite in respect of the murder of Mr Litvinenko.”