Douglas Carswell signed off a letter from condemning Ukip as a “one policy party” less than a month before he defected.
Giles Watling, the former Bread actor who is now standing for the Tories in Clacton, Essex, sent Mr Carswell a draft of a letter for approval which he was sending to the local paper.
The letter extolled Mr Carswell as a “strident Eurosceptic” and said that “a vote for Ukip will be a vote for Labour”. It said that Ukip’s attacks on the Tories were “madness” and that people should not be “fooled into voting Ukip”.
Mr Carswell replied in an email that the letter “reads very well” and that he would be “delighted” if Mr Watling sent it to the local newspaper.Less than a month later he defected to Ukip.
Details of the letter emerged ahead of the by-election in Clacton on Thursday, which the Conservatives are expected to lose.
However Mr Carswell said that “the significance is that I declined to sign a letter in my name attacking Ukip.
“I was decidedly cool towards the sentiments of the letter. My response was that of someone who clearly had doubts.”
The letter, which was subsequently published in a local newspaper, said: “The very idea that a UKIP candidate is standing against Douglas Carswell gives one the impression that this upstart party has no cogent thought of politics.
“They are a one policy party and yet they choose to attack a man who agrees on their one policy – madness!
“Since before UKIP was a glint in Nigel Farage ‘s eye Carswell has been a strident Eurosceptic – he would like to bring Britain out of Europe and has promised to hold David Cameron to his promise of an in/out referendum should the Conservatives form the next government.
“Don’t be fooled into voting for UKIP, they won’t and can’t deliver – a vote for UKIP will bring back Labour who put us in the mire in the first place, so the Conservative party must take UKIP seriously – no matter how difficult that might seem to be – for the sake of Britain!”
Mr Carswell replied: “That reads very well and I would be delighted if you were to send it to the press. Obviously, it is best coming from you – not me. Thanks so much and very well done.”
A Tory source said: “How can the public ever trust anything Mr Carswell says when he can’t be straight with people? This just shows Mr Carswell says one thing and does another.”
Some Conservatives are privately already conceding defeat in Clacton, where they expect to lose the seat to Mr Carswell by more than 6,000 votes.
If they fail to reduce Mr Carswell’s majority to 5,000, they are considering writing it off as a target seat in the 2015 General Election.
Ministers are instead likely to focus resources on the by-election in Rochester and Strood where Mark Reckless, another former Tory MP, has defected to Ukip.
Mr Reckless won the seat with a majority of less than 10,000 at the last election, and Cabinet ministers are confident that the Conservatives can win the seat and stem further defections.
By contrast Mr Carswell holds a majority of more than 12,000 and the former Tory MP is respected for his campaigning operation, which conservatives admit is “formidable”.