Zac Goldsmith has been branded a "rich boy playing fast and loose with the electorate" by a Tory MP furious the Conservatives are not contesting the Richmond Park by-election.
Mr Goldsmith will fight the election, scheduled for December 1, as an independent after he quit as Conservative MP in protest at the Government's support for a third runway at Heathrow Airport.
The Conservatives have said they will not field a candidate against Mr Goldsmith.
But Alec Shelbrooke, the Tory MP for Elmet and Rothwell, told Sky News his party was making a mistake by not selecting a candidate to battle for the seat.
He also attacked Mr Goldsmith's decision to resign.
He said: "I have said quite openly that I think it's a big mistake the Conservative Party is not putting forward a candidate.
"Let's be clear: Our majority will be cut from 12 to 10. Zac Goldsmith does not cut it to 11, it cuts it to 10 because he will be an opposition MP.
"The Conservative Party will lose the Richmond by-election because we do not have a candidate and I think that is wrong.
"The first time I think since 1963 we don't have a candidate and if Zac Goldsmith thinks that this campaign is just going to be about Heathrow, well, the Lib Dem candidate only has to pop up and say, 'yes, I also won't vote for Heathrow' and they can do that from the Commons and then it can be about a whole host of other issues.
"I'm afraid this really is a rich boy playing fast and loose with the electorate and actually a by-election costs the electorate hundreds of thousands of pounds."