David Cameron is finished whatever happens in the EU referendum

The Prime Minister wasn’t the only one to call the split in his party wrongly but he will be its first casualty

Instead of a small minority of Tory MPs arguing against the PM for Leave, it could well be half (Getty Images)

It is not often that you see a party of government splitting before your eyes. What is extraordinary is that the last time it happened, in 2003, we weren’t fully aware of it. When the House of Commons voted in favour of military action in Iraq all attention was on the strength of Tony Blair’s mandate across the House. With the support of the Conservative leadership, MPs supported the Government by a comfortable majority of 263.

We realised that 139 Labour MPs had voted against their own Prime Minister – the biggest parliamentary rebellion since 1846 – but we hadn’t fully appreciated that the Labour Party had split down the middle. It was a whipped vote: ministers voted for military action unless they were one of the few who resigned with Robin Cook. But exactly half of backbench Labour MPs voted against.

Unlike Iraq, there will be no vote in Parliament on David Cameron’s “reformed EU”. But the vote in the country on 23 June will be just as divisive and MPs will not be able to avoid declaring where they stand.

Blair stayed on in the top job for four years after the Iraq vote, so it wasn’t terminal, and David Cameron only wants to stay for another three and a bit. But the division now opening up in the Tory party doesn’t augur well.

I got the great split among Conservative MPs wrong. I thought that only a few more than the hard core of known Outers would oppose Cameron’s deal in the end. There were surprisingly few of these publicly declared anti-EUers until the past couple of weeks, not more than 30.

On the other hand, there were lots of MPs who called themselves Eurosceptics and who said they would wait to see what the deal was before deciding on In or Out. I thought they were like Cameron himself. He had long been impatient with the EU as a special adviser to ministers and thought of himself as a Eurosceptic, not least because he was staunchly opposed to Britain adopting the euro. For him, leaving the EU was not unthinkable. On the contrary, it was quite tempting. But he had never actually gone over the brow of that hill.

It turns out that a lot of Tory MPs are not like Cameron at all. They were not waiting and seeing so that they could stay in, they were waiting and seeing so that they could finally announce that they wanted to get out without automatically terminating their ministerial careers.

Instead of a small minority of Tory MPs arguing for Leave, it could well be half. The numbers are quite finely balanced: there are 330 Tory MPs, not including the Speaker, John Bercow. On Saturday the running tallies kept by Guido Fawkes and The Spectator had identified 142 of them as Outers. Some of them may be persuaded, as some of their Cabinet colleagues such as Sajid Javid have been. But some of the undeclared will join them – many of them are ministers and have so far been limited in what they can say. It looks as if the Leave total will be close to half of the Parliamentary Conservative Party. That’s 165 MPs.

This is not how Cameron hoped it would be. He thought his party – and the country – was looking for excuses to stick with the status quo. Instead, one of the unintended consequences of renegotiation has been to remind people who think of themselves as vaguely pro-EU of the things about EU membership that they don’t like. The European Parliament, the Brussels bureaucracy and Michael Gove telling us that “hundreds of new EU rules cross my desk, none of which were requested by the UK Parliament, none of which I or any other British politician could alter in any way”.

Oddly, most of Gove’s ministerial colleagues don’t share his frustration enough to want to turn their desks over. The split in the Conservative Party does not run in a perpendicular line. The top leans towards staying in the EU, MPs are split down the middle and the grassroots want to leave. That is the fault line with consequences.

If Cameron wins this referendum he will be hobbled by his party. Within moments of the result, the anti-EU Tory party will be looking towards the next referendum. At some point the EU treaties will have to be rewritten and it will be hard to resist demands for another referendum. Far from settling the European question, this referendum could ensure that Europe will dominate the Tory party’s choice of Cameron’s successor. Which is at least partly why Boris Johnson is making such an extended song and dance about his fence-dismounting: he wants to be the more Eurosceptic candidate if he faces George Osborne in the vote between the final two.

If Cameron loses the referendum, forget all his hints about staying on. His time would be over. His party would not countenance Brexit negotiations being handled by a leader who wanted to stay in. One way or the other, this is the end of his premiership: we just don’t know how or exactly when.

Twitter: @JohnRentoul