BREXIT is a “colossal misjudgement” that will diminish the UK’s standing in the world, harm the Special Relationship with America and could break up Britain, Sir John Major has said in a speech.

Delivering a lecture in London, the former Prime Minister launched a thinly-veiled attack on Boris Johnson, saying: “As a general rule, those whose focus is on self-advancement are rarely the most suitable to be entrusted with power.”

Stressing how he was free to give his honest opinion about what lay before the nation, he said: “There is no doubt in my own mind our decision is a colossal misjudgement that will diminish both the UK and the EU.

“It will damage our national and personal wealth and may seriously hamper our future security. It may even, over time, break up our United Kingdom. It will most definitely limit the prospects of our young.

“And once this becomes clear, those who promised what will never be delivered will have much to answer for; they persuaded a deceived population to vote to be weaker and poorer. That will never be forgotten nor forgiven,” the former Conservative leader declared.

He argued that without the anchor of the UK, the dynamics of Europe could change.

The UK would no longer, Sir John argued, be a “buffer between the Franco-German steamroller and smaller nations”.

He went on: “There is irony here: over 70 years ago, Britain stood alone to fight for Europe, now we freely choose to stand alone and, in so doing, undermine Europe.

“’So what?’ committed Brexiteers say, ‘we won’t be members; it’s Europe’s problem.’ But that ignores reality. How can it not be our problem, too?

“In the hot heat of debate it should not be forgotten that we ignore the EU, disdain it, or stand aside from it, at our own risk.”

The former PM also spoke about “good government,” which was a serious and complex business; far more important that who was up and who was down in the political stakes or slogans and soundbites.

Without mentioning the former Foreign Secretary by name, he stressed: “Most emphatically, it’s not about princelings fighting for the political crown of premiership; coded messages that shriek ‘I’m the one’ are about as subtle as a punch on the nose.

“Such self-interest is politics at its least attractive. It does not deliver sound government. It destabilises Government.

“As a general rule, those whose focus is on self-advancement are rarely the most suitable to be entrusted with power.”

The Tory grandee declared: “At this moment, our country needs to focus on policy, not personality; on substance, not show; on the national interest, not ideology. Because decisions that must soon be taken will shape the futures of our children and grandchildren for many years to come.”

On relations with America, he said Brexit would mean Britain’s value as an ally would decline.

“Be in no doubt: if the UK can no longer serve America’s interests in Europe, she will look elsewhere for someone who can…

“No ifs, no buts, we will be less relevant. No-one should be bedazzled by folksy talk of our Special Relationship; it is becoming less special year by year,” he insisted.

Sir John also bemoaned the polarisation of British politics, saying he preferred good old British pragmatism to rigorous ideology.

Politics is real life; it isn’t warfare…It’s about people; it’s about four nations who deserve more than an ideological tug-of-war.

“And the advocates of the extreme Right or Left must understand those with different opinions may well be opponents but they are still our countrymen and women. To treat them as ‘enemies’ or ‘saboteurs’ or ‘traitors’ is to poison both the political system and our way of life.

“Respect and civility would do much to help lift politics out of the dog days in which it is now living,” he added.