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Those David Cameron once called loonies, fruitcakes and closet racists will be savouring their revenge today.

UKIP has made a historic breakthrough in Westminster just a few months after winning a national election - for the European Parliament.

It has shown it can win under the first-past-the-post voting system as well as under a proportional system.

When Douglas Carswell takes his seat in the Commons next week he will serve not just as a permanent reminder of the prime minister's miscalculation but as a temptation to his old allies on the Tory benches to follow suit.

That is why the Conservatives are so desperate for victory in the next by-election in Rochester triggered by another Tory MP who defected to UKIP.

Ed Miliband cannot relish these Tory troubles. Although Labour held on to Heywood and Middleton, it did so with a vote share scarcely bigger than at the last election. That will do nothing to calm party nerves after a conference season that saw it go backwards.

The question now - how far UKIP will go? No-one knows but what is beyond doubt is that it has already gone much much further than those who dismissed and insulted it ever thought possible.