The spread-betting tycoon said it was "odds on" - and he urged those thinking about defecting to get in touch with him "for the sake of the country".
A UKIP source has also told BBC News the party is talking to five to 10 Labour MPs about possible defection.
But this was firmly denied by Labour, who accused UKIP of mischief-making.
Mr Carswell officially resigned as an MP on Friday, a day after he announced he was quitting the Conservatives and would fight a subsequent by-election as a UKIP candidate because he did not believe David Cameron was "serious" about reforming Britain's relationship with Europe.
The prime minister described the move as "slightly bizarre", telling reporters that Mr Carswell was one of those who wanted to leave the EU irrespective of the outcome of negotiations he was confident would lead to a better deal for Britain in Europe.
"He fought as a Conservative in 2010 when we didn't have a commitment to an in-out referendum and he has left the Conservative Party at a time we do have a commitment to an in-out referendum - that is a question for him to explain rather than me."
Mr Carswell's defection prompted a flurry of speculation that other Eurosceptic backbenchers would jump ship - fuelled by Mr Wheeler's revelation that he had discussed the possibility with eight of them in recent years, although he declined to name names.
He told BBC News he had approached 10 Tory MPs and eight had agreed to meet him secretly for lunch at his favourite Italian restaurant in Mayfair.
They did not include Mr Carswell, he revealed, adding that UKIP leader Nigel Farage had handled that defection personally, and he had not always directly asked them to defect, just inquired whether they wanted to meet Mr Farage.
Asked by BBC Radio 4's Becky Milligan, if any of them had been tempted to defect, he said: "I am sure they will be tempted but the question is whether they give into that temptation."
Asked about the likelihood of other defections before the general election, he said: "If I had to put money on, one way or another, before I knew the result of this by-election, I would say odds on, more likely than not but only just."
MPs wanting to discuss their future should contact him in the strictest of confidence, added the former Tory donor.
"For the sake of the country they should defect because we are doing the right thing for the country and the Conservative Party are doing the wrong thing," he told BBC Radio 4's The World at One.
A senior UKIP source also told Becky Milligan that the party was talking to five to ten Labour MPs about possibly changing affiliation.
The MPs in question, the source said, were "deeply unhappy with their party, and feel that people are fed up being patronised by the Labour glitterati".
But Labour firmly denied any of its MPs were in talks with UKIP or considering defection.
Shadow defence secretary Vernon Coaker said: "I'm not about to jump and you won't see other Labour MPs jumping."
He said he was not aware of any Labour MPs who had sufficient doubts about Labour's stance on Europe to look elsewhere.
Mr Farage, who visited Mr Carswell's constituency with his new colleague, said he was "constantly" talking to Eurosceptics in the Tory and Labour Party.
Mr Farage told reporters: "There is only one party for a genuine Eurosceptic. That is UKIP."
He said talk of further defections was "speculation", but added: "If Douglas Carswell wins this by-election, and I believe he will, there will be others on the Tory and Labour backbenchers who may think 'UKIP is for us.'"
Prominent backbenchers Peter Bone, Stewart Jackson and Nadine Dorries are among those to have ruled out a defection to Nigel Farage's party as Eurosceptic Tory MPs rallied round David Cameron.
The seaside seat of Clacton's population is older than the national average, and pollsters have found that UKIP's appeal is strongest among the over-60s.
Analysis of demographic data by Nottingham University's Matthew Goodwin suggests that Clacton is "the most favourable seat for UKIP in the country".
In graphics: Will Clacton's voters opt for UKIP?
Conservative Eurosceptic John Redwood said talk of eight possible defections was a "figment of UKIP's imagination", suggesting that this was merely an estimate of the number of MPs who may have had lunch with Mr Wheeler.
"Dream on UKIP. It is the kind of figure that you would put round if you were UKIP." he told Radio 4's Today programme.
Among MPs to have ruled out joining UKIP include long-standing EU rebels Mark Pritchard, John Baron, Bill Cash, Bernard Jenkin and Jacob Rees-Mogg.
Speaking in Clacton, Mr Carswell said he had a "phenomenal amount of support" since announcing his decision, adding there had been a "real shift in opinion locally".
He added: "I could lose. There is a risk in this. I am putting my principles on the table."
A local councillor in Sevenoaks, Kent, has become the first elected representative to defect from the Conservatives to UKIP since Mr Carswell's decision.
Chris Neal, who represents the Cowden and Heaver ward on Sevenoaks District Council, said he felt the Conservatives "no longer have the interests of ordinary people at heart" and their position on the EU was "an insult to democracy".
He said he had considered "staying quiet" until next May's local elections, when he plans to step down as a councillor, but had been inspired to switch sides by Mr Carswell.
Further Conservative MPs are "more likely than not" to follow Douglas Carswell to UKIP, the party's treasurer Stuart Wheeler has told the BBC.