Vladimir Putin’s seizure of Kyiv would be a “catastrophe” bigger than Vietnam for American leadership on the world stage, Boris Johnson has warned.

The former British Prime Minister said he was “praying” Republicans in Congress would end the blockade of around $60 billion in funding for Ukraine, saying: “the situation is grim and the delay is terrible”.  

Mr Johnson went on to warn that the Russian president’s ultimate goal was to “get Kyiv”. “He wants to take control of the whole country,” he said.

He continued: “If that happens, I think it would be an absolute catastrophe for the West, and a catastrophe for America and for any concept of American leadership.”

“I think it would be a reverse bigger than Vietnam,” he said, “because this would be Nato, after 75 years of success, that had failed for the first time in a European war”.

He added that the West’s chaotic 2021 “retreat from Afghanistan will be nothing compared to Russia winning in Ukraine. Nothing.”

Mr Johnson went on to express optimism that Donald Trump would abandon his isolationist rhetoric over Russia’s war in Ukraine should he regain the White House in November’s election.

“I really can’t believe that any American president or incoming American president is going to want that to happen on his watch,” Mr Johnson said.

Mr Trump, 77, has previously said he would encourage Russia to do “whatever the hell they want” to any Nato member that doesn’t meet its defence spending commitments.

But Mr Johnson said: “When you look at the substance of what Trump did in the White House on foreign policy, you can make a case for quite a lot of it”.

Speaking to reporters later, Mr Johnson stressed that his suspicion was that Mr Trump would not abandon Ukraine to Russia.

The former Prime Minister, 59, was speaking to an audience at the Center for European Policy Analysis, a Washington think tank.

He was introduced, in accordance with American custom, by the moderator as “Prime Minister Boris Johnson”.

“I love the way you call me Prime Minister. It’s a courtesy that doesn’t exist in the United Kingdom,” he said, joking “it might come in handy”.

During the more than hour-long “fireside chat”, Mr Johnson argued that there was no feasible peace deal the Ukrainians could strike to end the fighting.

He said: “There’s no deal they can see they can do with Putin. I mean, nobody comes up to me furtively and says, ‘oh, but you know, by the way, we could trade this and this and let’s cut a deal’. Nobody suggests that.”

However, Mr Johnson argued there are ways through for Putin to “end this thing” and still be able to “claim some sort of moral victory”.