Clacton By-Election Will Be On PM's Birthday
The Clacton by-election will be held on October 9, Prime Minister David Cameron's birthday.
Douglas Carswell, the sitting Tory MP, defected to UKIP last week and announced he would fight for his seat as a candidate for his new party.
The MP said he made the decision because he believes UKIP is the only party that can break up the "cosy Westminster elite".
Nigel Farage praised his newest recruit, telling him: "I think what you've just done is without doubt the bravest, most honourable and noblest thing I've seen in British politics in my lifetime."
A by-election win would signal a "landslide in British politics", Mr Farage said.
But Mr Cameron said the decision was "deeply regrettable" and "self-defeating".
Conservative MP Bill Cash - a standard-bearer for the eurosceptic cause - also criticised Mr Carswell's decision, calling it a "monumental misjudgement".
The electoral test will come a week after the Tories' annual conference, and an opinion poll at the weekend suggested the party is on course for defeat.
The Conservatives have yet to announce who their candidate will be.
Mayor of London Boris Johnson has brushed off suggestions he should put himself forward.
He told LBC radio: "The first objection really, is that I've already whacked in the CV for Uxbridge. It would seem a bit eccentric."
Mr Carswell has been an MP in the Essex seat since 2010 and has a majority of 12,000.
(c) Sky News 2014