Douglas Carswell’s defection today to Ukip is terrible for David Cameron. But it is also deeply inconvenient for his band of eurosceptic brothers. He was a key member of a powerful ‘cell’ of MPs who met regularly to discuss strategies for pushing the Conservative leadership further on European policy.

One key colleague in this cell tells me that its members are as shocked as anyone else by the defection because ‘Douglas was refusing to get involved in our shenanigans. It was difficult to get him to sign off on anything we wanted to do, he was incredibly loyal, so something serious must have happened over the summer to change his mind.’

Carswell certainly hinted at conversations with the Prime Minister’s advisers that may have changed his mind. He had declared his loyalty to Cameron after the Prime Minister said he would pursue the EU referendum bill (#letbritaindecide). But he said today that the advisers had said they would only get what was sufficient in terms of reform, rather than being ambitious. This illustrates the danger for the Prime Minister of setting out any detail of what he wants from a renegotiation of Britain’s relationship with Europe, but as I say in this week’s Spectator, it is that detail, rather than a new threatening tone, that other eurosceptic MPs want.

But what happens to that eurosceptic cause now? They had been discussing how to get more detail out of the Prime Minister, and presumably Carswell had been a block to any ‘shenanigans’ on this. But just because he has left the building and seriously shaken up the Tories, it doesn’t mean that Carswell’s defection is helpful to the eurosceptic cause. ‘He was one of the Bolsheviks,’ says his campaigning colleague. ‘And it’s going to be more difficult to push the eurosceptic line in our party.’