Conservative grandee John Redwood has dismissed the "so-called" eight Tory MPs allegedly considering defection to Ukip as "figments of Ukip's imagination".
"Dream on Ukip," the eurosceptic MP for Wokingham and former Wales Secretary told Radio 4's Today programme.
"I treasure the words of Mr Carswell which he was speaking yesterday. He said that we need to deliver an in-out referendum, he said it would only happen if David Cameron was prime minister," Mr Redwood said.
"He said we must do nothing to make the prospect of a referendum less likely. He was super-loyal until yesterday."
Ukip's general election candidate for Clacton has vowed to fight Douglas Carswell for the party's nomination in the upcoming by-election.
Roger Lord said he would "absolutely" fight for the nomination. "I have been assured I will get a fair hearing," he told Radio 4's Today programme.
Asked whether he would step aside to make way for Mr Carswell, Mr Lord replied, "We are not a party that stands down. We don't bend, we don't break. We fight."
Mr Lord also accused Mr Carswell of jumping ship from the Conservatives because the water was "around his ankles".
Nigel Farage has hinted that more MPs could follow Douglas Carswell by defecting to his party before the election.
The Ukip leader said politicians among the Conservative and Labour ranks "feel strongly" on the issues the party stands for.
"I don't think anybody will say that Douglas Carswell coming to Ukip is a one-off.
"There are many other people out there that are making up that calculation now," he added.
Douglas Carswell will return to his Clacton constituency today flanked by Ukip leader Nigel Farage as the pair plan a completely unexpected by-election campaign.
They are due to visit the seat after Mr Carswell's bombshell decision to switch parties and quit the Commons to re-stand under the Ukip banner.
The date for the by-election is yet to be set but is likely to be held within weeks. Parliamentary rules require an election between 21 and 27 days after a writ is moved to fill the vacancy.
A disgruntled UKIP candidate has spoken out after party leaders passed over him to back defected Tory MP Douglas Carswell instead.
Mr Carswell, the MP for Clacton, sparked a by-election after defecting from the Tories to UKIP.
And despite already selecting Roger Lord to fight the seat in next year's General Election, UKIP has announced it will be backing Mr Carswell in the pending by-election.
Speaking to BBC Essex, he said:
It is an enormous discourtesy, for anybody really, just to announce that and say that.
I think the last time I saw him, I said I would be prepared to give him a reference come next May if he found himself unemployed, which I am pretty sure he would be.
Perhaps he is jumping ship to try and get in front of all the other Conservative MPs who are going to find themselves in the unemployment queue come next May.
A UKIP spokeswoman said by-election candidates were selected differently to General Election candidates, and said she hoped he would "understand" the decision.
Prime Minister David Cameron has described Tory MP Douglas Carswell's defection to UKIP as "deeply regrettable" and "self-defeating".
William Hague has said Douglas Carswell's decision to defect to Ukip is "totally counter-productive" because only the Conservatives can offer the public a choice on EU membership.
The Leader of the Commons said: "It's a totally counterproductive thing to do, of course, because the only chance of real change in Europe, the only chance of having a referendum where people can decide to say in the EU or leave the EU is having a Conservative government."
Backbench Tory MP Mark Pritchard agreed with Mr Hague, saying Mr Carswell had made an "error of judgment" that would end up helping Labour in the 2015 general election.
Defecting Conservative MP Douglas Carswell says he want to "shake up" the Westminster "clique" with his move to Ukip.
Carswell said the Conservatives' failure to take a stand against the European Union was at the heart of his decision.
ITV News Political Correspondent Libby Wiener reports:
Ed Miliband has tweeted that Douglas Carswell's defection show the Conservatives are "too divided to stand up for hard-working families".
Douglas Carswell’s defection is not just a blow to David Cameron. It shows the Tory Party too divided to stand up for hard-working families
A senior Tory in Douglas Carswell's Clacton constituency has said news of his defection to Ukip has come as a "bolt from the blue".
Mick Page, the leader of Tendring District Council, which covers Clacton said: "That's come as a shock - a bolt from the blue."
Speaking to the Press Association he added: "We always thought he was a Conservative very much against Europe but we thought he would work within the party to achieve his aims. As far as I'm aware the Conservative association had no warning of this."