David Cameron has compared Vladimir Putin to Adolf Hitler as he called on the US Congress to approve a multi-billion dollar funding package to Ukraine.
The foreign secretary said he was “going to drop all diplomatic niceties” in a direct plea to American politicians.
President Joe Biden wants to send $95.3 billion (£75 billion) of support to Ukraine to help it continue its war against Russia.
In an unusual move, Lord Cameron used an article in Washington newspaper The Hill to urge Congress to support it.
He said: “We face a choice. A simple test.
“On the one side is Putin, hoping to enlarge his empire simply by outlasting the West. He believes we are weak. He believes he can get away with the most shocking act of national aggression we have seen in our lifetimes.
“On the other side are all of us. We have the resources, the economic might, the expertise. Our economic strength outweighs Russia’s by a factor of around 25 to one. They are having to turn to Pyongyang for help. All we need to do is make our strength pay. The question is: Do we have the will?
“As Congress debates and votes on this funding package for Ukraine, I am going to drop all diplomatic niceties. I urge Congress to pass it.”
The foreign secretary added: “I believe our joint history shows the folly of giving in to tyrants in Europe who believe in redrawing boundaries by force.
“I do not want us to show the weakness displayed against Hitler in the 1930s. He came back for more, costing us far more lives to stop his aggression.”
Lord Cameron admitted that the West had failed previous tests against Putin - including when he was still prime minister in 2014 and Russia annexed Crimea - but said it must not falter this time.
“It goes to the heart of what both sides of the aisle stand for. What both our countries stand for,” he said.
“We fight aggression. We stand up for freedom. We stick by our friends. We show this dangerous, uncertain world that we are unbending in our will. And we win.”