Image copyright AFP/Getty Image caption There will be speculation about who else could join Douglas Carswell in defecting to UKIP

This is a body blow for David Cameron.

When he became Tory leader, he told his party that they needed to stop obsessing about Europe.

This defection - and the by-election which will follow - will revive that obsession.

Conservative MPs and activists, as well as the Tory press, will angst and speculate about who might follow Douglas Carswell into UKIP's ranks.

They will debate and argue about whether their leader now needs to threaten to leave the EU and spell out his full negotiating demands. They will disagree and fall out about whether and how to control immigration.

Their coalition partners in the Lib Dems will do nothing to ease their plight.

Labour will highlight every Tory split as evidence of their unfitness to govern and portray every utterance from Number 10 as a lurch to the right or of policy-making dictated by Nigel Farage.

The Scottish National Party will tell Scots that the only way to opt out of all this is to vote to break up the United Kingdom.

The earliest the by-election can be held is early October, immediately after the Tories' big pre-election party conference - just when they needed to focus on the battle against Labour and Ed Miliband.

Media playback is unsupported on your device Media caption Mr Cameron said the resignation was "deeply regrettable"

Whenever it is actually held, almost certainly much later, it will hover like a cloud over the Conservatives.

William Hague, Cameron's predecessor and the man he calls his real deputy, once compared the issue of Europe to an unexploded bomb.

Not any more. It just went off.

PS

Carswell's defection genuinely shocked even his Eurosceptic friends in the Tory party - in large part because he had praised Cameron for promising an EU referendum and for his Bloomberg speech spelling out his strategy of renegotiating Britain's relationship with the EU.

Tory HQ have already dug out a series of quotes which they will deploy against the man who is now UKIP candidate for Clacton.

Here are a couple of them: