Sir John Major has said Brexit will transform Britain into a “middle-sized, middle-ranking nation” and those who argued for it “will never be forgotten - or forgiven”.
In a major intervention by the former Prime Minister, Sir John also said Brexit was a “colossal misjudgement” which could lead to the break up of the United Kingdom.
He said leading Vote Leave politicians, such as Michael Gove and Boris Johnson, had “persuaded a deceived population to vote to be weaker and poorer”.
It comes just hours before Prime Minister Theresa May is set to head to Brussels to meet with EU leaders for a crucial summit.
Delivering the Michael Quinlan Lecture in Central London, he said: “We are told our future aim is to be ‘Global Britain’: that is certainly the right policy, but it is hardly new. It has been the reality for 300 years.
“What is new is that much of the world will now perceive Britain to be a middle-sized, middle-ranking nation that is no longer super-charged by its alliances. Suddenly, the world will be a little chillier.”
The senior Tory, who campaigned for remain, said that leaving frontline politics had allowed him to consider Brexit talks outside of party lines.
That will never be forgotten – nor forgiven. Sir John Major
He said he sympathised with Leave voters’ frustrations with the European Union, but added: “Nonetheless, after weighing its frustrations and opportunities, there is no doubt in my own mind that 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 – I believe 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.”
He added that the financial crash had “destroyed” faith in politics and said that Britain leaving the EU would be destabilising for the Continent.
“Without the UK, the dynamics of Europe may change,” he said.
He added that being isolated could be damaging, saying: “We live at a time when America is showing withdrawal symptoms, and China is growing in economic, political and military power. Whenever the US leaves a vacuum around the world, it will be filled by China, or Russia, or regional players.
“Already, Russia is a far more significant presence in the Middle East than would have seemed conceivable a decade ago.
“The fundamental point is simple: if America withdraws from international obligations, then Europe can best protect her own interests if she is united.”