Ukip leader Nigel Farage has hinted his party could gain more defectors after a Tory MP caused shockwaves by joining the arch eurosceptic party.
As he announced his new allegiance, Douglas Carswell accused David Cameron of not being serious about EU reform.
Mr Farage added that it was "no great secret" that a number of both Tory and Labour MPs agreed with Ukip "very strongly".
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And another Tory MP suggested that "10 to 15" of his colleagues had also considered such a move in recent months.
There was widespread surprise at Mr Carswell's defection - and his announcement that he was immediately resigning from Westminster to refight his seat for Ukip.
Bookmakers immediately backed him to take the constituency of Clacton in Essex comfortably. He currently has a majority of more than 12,000 and has been the MP since 2005.
Mr Carswell said he had to do the decent thing and ask the electors for his job back.
At a hastily arranged press conference he appealed to voters to back him, saying: "Let's see if we can make history".
The by-election will create a big headache for Mr Cameron, who will have wanted to avoid a Tory versus Ukip battle months before next year's General Election.
The SNP last night predicted the vote would cause Westminster to turn in on itself and obsess over euroscepticism.
SNP Westminster leader Angus Robertson said the defection marked "yet another step by Westminster towards the European exit door."
He added: "The Westminster establishment are dancing to a damaging Ukip agenda - threatening to rip Scotland out of Europe against our will - which is one of the factors causing people to make up their mind in favour of a Yes vote next month."
William Hague and other Tories said Mr Carswell's move was "counter-productive".
Mr Farage hailed Mr Carswell's defection as the "bravest, most honourable and noblest" move he had seen in politics and said they had discussed it 18 months ago.
Last night Mr Cameron said his party would fight as hard as possible to retain Clacton.
He added: "It is obviously deeply regrettable when these things happen and people behave in this way. But it is also, in my view, counter-productive. If you want a referendum on Britain's future in the EU, whether we should stay or go, the only way to get that is to have a Conservative government after the next election."
Last night a complication emerged as Roger Lord - the already selected Ukip candidate for Clacton - refused to stand aside.