Alexander Temerko has asked the prime minister to reconsider his objections to a no-fly zone, arguing it could save the lives of millions of people.

The veteran Tory donor, who gained British citizenship in 2011, argues the threat of nuclear retaliation by Russia is low because the generals and colonels who carry out the instruction would not obey.

However, he said that the US and UK should at least consider a no-fly zone over western Ukraine if they do not think it is feasible to protect the whole country.

He told Sky News: “Putin understands that he can lose his army in western Ukraine. And he will assault and bomb Ukraine with missiles.

“That will be a huge humanitarian catastrophe if it happens next week.

“And today, President Zelenskyy and the Ukrainian diaspora across the whole world call on governments to please use their authority to make a decision to close the airspace around Ukraine.

“If you cannot create a non-flight area across Ukraine, do it in the western part of Ukraine. That is a drastic and vital decision to save the life of millions of people.”

“Putin can give the command,” he said. “It’s possible because he’s insane and an unpredictable person, very poorly informed.

“But I don’t believe that at the middle level – generals and colonels – would fulfil or perform his order. Most of them have a family, they love their family, they love their life, and they understand consequences.”

However, he stopped short of calling Mr Putin evil: “I think he’s insane, not evil. He is paranoid and far from reality.”