Speaking to Sky News' Beth Rigby Interviews programme, Mrs Litvinenko said the UK should "absolutely" introduce a visa waiver for those seeking sanctuary from the war in Ukraine.

Mrs Litvinenko - whose husband Alexander was poisoned with radioactive polonium in London in 2006 - stated that she didn't think Britain was welcoming enough Ukrainian refugees.

"We know these pictures from Calais and Paris where people need to go back to apply for visas," she said.

"It's a shame. Because I saw these people spend hours and hours to get through the Ukrainian border and then go to see, maybe, their relatives here in the UK.

"I would like to say, these people don't want to get out from Ukraine, they are being pushed from Ukraine.

"They're not looking for a better life, they will definitely come back [to Ukraine] when it [the war] will be all finished."

Earlier on Thursday, Home Secretary Priti Patel announced she was cutting some of the red tape for Ukrainians applying to join family members already in the UK.

However, she defended the continued need for Ukrainians to apply for visas before coming to the UK due to the "important security checks" that need to be done.

Charities have called for the Home Office to abandon the requirement for Ukrainians to apply for visas, at least for a temporary period, in line with other European countries.

Mrs Litvinenko, who recently returned to the UK from western Ukraine, revealed to Sky News that one of her female friends - a member of Ukraine's parliament - was currently fighting in Kyiv with a Kalashnikov rifle.

She said her late husband, a former FSB officer who became a prominent critic of the Kremlin, had known Mr Putin was bad for Russia from the "very beginning".

Mr Litvinenko's murder is suspected to have been personally signed off the Russian president.

The Russian people have been left "completely brainwashed" by Mr Putin's two decades in power, Mrs Litvinenko suggested.

"Some people are just afraid to think about the truth," she told Sky News. "They maybe understand something very bad is happening in Ukraine, but they are afraid to talk about this.

"You need to accept you live in the country who are the aggressor and this is very difficult."

Mrs Litvinenko added that she didn't know if the Russian people might overthrow Mr Putin following his attempt to invade Ukraine.

"The Soviet Union collapsed in 1991 when nobody believed it," she said, adding that modern Russia was "not as strong" as the Soviet Union.

"Yes, they try to control everybody, every individual, but they still have people outside in the street at protests."