Until now, every US president I have known has considered [the UK’s] relevance to America to be enhanced by our membership of the European Union. Yet very soon – on our current course – we will no longer be able to argue from within the EU for Anglo-American beliefs in free trade, open markets and strong defence. Our friends, the Americans, are hard-headed about power. It is romantic folly to think otherwise. Be in no doubt – if the UK can no longer serve America’s interests in Europe, she will look elsewhere for someone who can.

Our value – as an ally of America – will decline. 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.

For centuries, our state schemed and plotted to prevent all Europe uniting against us. Now, we have chosen to turn our back on all Europe. A long line of former statesmen will be turning in their graves.

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.

If the art of negotiation is to obtain what you seek, then the intention must be to give a little to (hopefully) gain a lot more.

I cannot know how the government has conducted negotiations in private with the European Union: very possibly they have met the tenets I have set out. But, even if they have (and not all the signs are good), belligerent noises-off – on a daily basis – have built up ill will, and made the prime minister’s task even more difficult.

We know the post-Brexit world will be very different from now. It cannot be otherwise, because no form of Brexit will remotely match up to the promises made by the leave campaign in the referendum: they were vote-gathering fantasies, not serious politics.

I have no constituency vote clouding my view of Brexit. I have no ambition driving my support for it. I have no party whips demanding loyalty before conscience. I have made no false promises about Brexit that I must pretend can still be honoured, even though – in my heart – I know they cannot. I am free to say absolutely and precisely what I believe about Brexit. And it is this:

I understand the motives of those who voted to leave the European Union: it can – as I well know – be very frustrating. Nonetheless, after weighing its frustrations and opportunities, there is no doubt in my own mind that our decision is a colossal misjudgment 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.

Our domestic focus is on the impact leaving Europe will have on the UK. That is quite natural, but, to the world at large, the bigger question is how the EU itself will be affected. The answer is – badly. Without the UK the balance of the EU changes. The free market majority may be at risk: protectionists will be encouraged and, perhaps, empowered.

The UK will no longer be a buffer between the Franco-German steamroller and smaller nations. Germany will be more isolated, and friction may grow. “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?

Whether we are “in” or “out” the EU is in our neighbourhood; it is our predominant economic partner and our wellbeing is inexorably linked to their own wellbeing. 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.

None of the mainstream political parties is in a healthy condition. Both the Conservatives and Labour face pressure from fringe opinion within their own membership.

My fear is that the extremes of right and left will widen divisions and refuse to compromise, whereas more moderate opinion will often seek common ground. The risk of intransigence – “my way or no way” – is that the mainstream parties will be dragged further right and further left.

Our nation should not tolerate the unreasoning antipathy of the extremes – to the EU, to foreigners or to minority groups. Such antipathy is repellent, and diminishes us as a nation. Softer, more reasonable voices should not be drowned out by the raucous din of the loudest.

I freely confess to a taste for compromise. Politics is real life. It isn’t warfare. It isn’t a popularity contest. It’s about people. It’s about four nations who deserve more than an ideological tug of war.

Respect and civility would do much to help lift politics out of the dog days in which it is now living. More compromise – less confrontation.

• John Major is a former British prime minister. This article is an extract from the Michael Quinlan Lecture, Compromise or Confrontation, delivered by Major in London on Tuesday 16 October