Mr Johnson distinguished between the two “very different” cases. “Poland has a Nato security guarantee and under Article 5 we’re committed to the defence of Poland,” he said.

“Ukraine, sadly for historical reasons, does not have the same guarantee from Nato powers. Ukraine doesn’t even have a membership action programme,” Mr Johnson added.

The Prime Minister declared that “what we’ve got to do is to make sure that everybody understands the cost of miscalculation on the borders of both Ukraine and Poland would be enormous”.

Provoking Russia

It comes after the former defence secretary Michael Fallon wrote in The Telegraph that he had made the case as far back as 2014 to arm the Ukrainian Army, but had been denied the chance by the cabinet.

Mr Fallon wrote: “We gifted some protective equipment, but my coalition cabinet colleagues wouldn’t go further: they didn’t want to ‘provoke’ Russia. I was restricted to providing ‘non-lethal assistance that will reduce fatalities and casualties’.”

However, he said that the compromise of Operation Orbital, which saw British troops train more than 20,000 Ukrainian troops was not enough, as he called for the UK “to build up defensive capability and resilience right across Ukraine”.

Defence secretary, Ben Wallace, and his Ukrainian counterpart, Oleksii Yuriyovych Reznikov, said in a joint statement: “Our governments have no desire to be adversarial, or seek in any way to strategically encircle or undermine the Russian Federation.”