The Labour split on immigration is well documented. The party is experiencing its own class war, with many working class supporters demanding a reduction and middle class supporters typically more supportive. What's been less commented on is the extent to which immigration also presents an existential threat to the Conservative party.

Look at Boris Johnson's Telegraph column last night. It contains all the clues for what will happen to the Tory party over the next few months.

On a policy level, it makes no sense. British people will still be able "to live; to travel; to study; to buy homes and to settle down" in Europe. We'll still be in the single market. But there'll simultaneously be "a balanced and humane points-based system".

That's impossible. You can have access to the single market and freedom of movement, or you can have neither. There is no in-between option.

People have tried. Reform of freedom of movement was the first thing David Cameron requested when he knew he'd hold a referendum. He tried to do it before he even made his speech announcing the referendum. Europe wasn't having it then and it won't have it now.

So given that Boris' policy proposals are impossible, what are we to make of the column? All we're left with is tone – and the tone strongly suggests we're going to keep freedom of movement. Johnson says the winners of the referendum "must accept that it was not entirely overwhelming". He goes on: "It is said that those who voted Leave were mainly driven by anxieties about immigration. I do not believe that is so."

It reads as a pretty clear effort to prepare the ground for a substantial U-turn. There's no proof of course. The article is internally inconsistent – a list of things he'd like without any recognition that having some of them rules out having others.  It's a poem to unicorns. But tonally it seems clear which direction he's pointed.

Anyone in his position would do the same. Leaving the single market would be an economic catastrophe for Britain. It is the abyss. Writing new trade deals with EU partners in an Article 50 negotiating system designed to give them overwhelming power over you is not going to result in a positive outcome. And that goes double when you also have countless reams of legislation to unpick, as well as the political matter of extricating yourself from the EU. All that in two years. It would be an unmitigated disaster.

And that's not even to mention the economic cost. A two-thirds reduction in immigration would shrink the size of the economy by nine per cent by 2065. Even with immigration of 140,000 a year (about half of where it is now) our debt will reach 99% of GDP in 50 years. If we cut immigration altogether, it'll be 174%.

No-one with half a brain will go down that road willingly. And Johnson does have half a brain. So he won't.

Where does that leave the Tories? As savaged by immigration as Labour is. Many, if not most, of those who voted Leave will feel utterly betrayed by a politician who says freedom of movement will continue. It will make Nick Clegg's tuition fees pledge look like a children's cartoon. It will be seen as a historic betrayal.

And who will stand to benefit? Who will be over by the side, holding the 'Real Brexit' banner? Nigel Farage. He will have found another reason to exist.

It will tear the Conservatives apart as badly as Labour is being torn apart and on precisely the same issue. We are not just witnessing what could be the moment of Labour break up – we are looking at a process which could easily end up doing the same to the Tory party.

When Cameron made that referendum speech, he really had no idea what he'd unleashed.

Ian Dunt is the editor of

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