Tory chairman Grant Shapps has accused Mark Reckless of "betrayal" with his defection from the party to UKIP.
Mr Shapps told the party conference in Birmingham: "Your trust has been abused. You have been cheated."
Mr Reckless announced he was defecting to UKIP on the eve of the Conservative conference, triggering a by-election in his Rochester and Strood constituency.
He said it was "impossible" to keep his promise to voters on tax cuts and Europe as a Conservative.
David Cameron has called Mr Reckless's defection - which follows a similar move earlier this month by MP Douglas Carswell - "senseless and counter-productive" because the Conservatives were the only party that could deliver an EU referendum.
But in his opening speech to the Conservative conference, Mr Shapps went further, telling members: "I share your deep sense of betrayal and anger. We've been repeatedly let down by someone who lied to his constituents and you.
"He lied, lied and lied again."
David Cameron wants to use his week in Birmingham to unveil a series of policies aimed at winning the party an overall majority in May's election.
He told BBC One's Andrew Marr Show that a Conservative government would cut the maximum level of benefits a household can claim from £26,000 a year to £23,000 to fund an extra three million apprenticeships in the next parliament.
At the same time, unemployed young people aged 18 to 21 would be given six months to find work or training - after which their jobseeker's allowance payments would be withdrawn unless they agreed to take part in "community projects" such as cleaning up local parks.
A further proposal is that most unemployed 18 to 21-year-olds should also be prevented from claiming housing benefit, if the claim is being made solely to enable them to leave the parental home.
Mr Cameron also promised help for younger people to get on the housing ladder, saying a Tory government would build 100,000 homes exclusively available to under-40s in England at 20% below the market rate.
But the policy announcements have been overshadowed by the defection of Mr Reckless and the resignation of civil society minister Brooks Newmark over newspaper revelations about his private life.
Mr Reckless, who spoke on Saturday of his wish to cut immigration, told BBC One's Sunday Politics show: "We made all these promises in 2010 as Conservatives and they have been broken.
"David Cameron has had his chance, he hasn't kept his promises. I want to keep mine."
At the scene - Mike Sergeant in Rochester
On a walkabout in his constituency town of Rochester, Mark Reckless was challenged about recent assurances he had given to party members that he would not be switching to UKIP. He said: "You can't announce these things in advance... what's important to me is keeping my promises to my constituents and it's easier to do that in UKIP."
Mr Reckless was on a walkabout with Nigel Farage the day after announcing his decision to join UKIP. The men went to The Crown pub on the high street. Mr Farage had a pint of bitter; Mr Reckless restricted himself to orange juice.
During the walkabout, members of the local Conservative party said they tried to ask the MP questions but were prevented by UKIP security. Several members of the local party - who gathered outside the pub - said they felt "betrayed and let down". But the mood remained calm throughout.
UKIP leader Nigel Farage said: "There is a great deal of disenchantment with David Cameron's leadership of the Conservative party but equally don't underestimate the fact that there are Labour backbenchers who are thinking about this as well.
"It wouldn't surprise me if we saw more defections."
Mr Cameron said he had not been aware of Mr Reckless's plans to quit but suggested the backbencher would not be much of a loss to the party because he "rarely voted with the government".
He told the BBC's Andrew Marr show that the choice at the next election was between Labour and the Conservatives, "so to act in a way that makes the Conservative government less likely is senseless and counter-productive."