Stuart Wheeler, the millionaire Ukip donor, claimed he had wined and dined some eight backbench Tory MPs to encourage them to consider switching.
"I didn't say, would you defect? I would say, would you like to meet Nigel Farage? Some of them said yes and obviously Douglas Carswell was one of them," said Mr Wheeler.
But John Redwood, the former Welsh Secretary, branded talk of eight possible defections a "figment of Ukip's imagination", suggesting this was merely an estimate of the number of MPs who might have had lunch with Mr Wheeler. "Dream on Ukip," said Mr Redwood.
A senior Ukip source claimed the party was talking to up to 10 Labour MPs about defection. Vernon Coaker, the Shadow Defence Secretary, said: "I'm not about to jump and you won't see other Labour MPs jumping."
David Cameron described Mr Carswell's decision as "bizarre".
The Prime Minister said: "His decision is slightly bizarre because he fought as a Conservative in 2010 when we didn't have a commitment to an in/out referendum but he has left the Conservative Party at a time when we do have a commitment to an in/out referendum. That is a question for him to explain, perhaps, rather than me."
Mr Carswell joined Mr Farage for a walkabout in his former Essex constituency and defended his decision to quit, saying: "I stab people in the front, not the back."