Conservative deputy chairman Howard Flight has been forced to quit after he suggested the party's true spending plans had been hidden from voters.
Mr Flight says he 'regrets' his remarks
In a tape obtained by The Times, Mr Flight tells Tory activists more cuts were in the pipeline but the party "had to win an election first".
A Tory spokesman said Mr Flight's views did not represent party policy.
Labour said it proved the Tories planned "devastating" cuts. The Lib Dems said Tory figures do not add up.
Mr Flight, a key architect of Tory economic policy, was involved in setting up the James review into public spending, which identified £35bn of savings.
But he told a private party meeting on Wednesday: "The potential for getting better taxpayer value is a good bit greater than the James findings (which have been) 'sieved' for what is politically acceptable and what is not going to lose the main argument."
He hinted that further tax and spending cuts would be possible once the Conservatives were in power because "everyone on our side of the fence believes passionately that it will be a continuing agenda".
Mr Flight said after an election had been won, "you can actually get on with what needs to be done".
According to the Times, Mr Flight described the Tory proposal for pensioners' council tax rebates as "nakedly political".
He suggested further vote-winning promises were likely to include raising income tax and inheritance tax thresholds "so that Middle England comes out of the equation for the time being".
Mr Flight, a banker and former advisor to Norman Tebbit, said he regretted his choice of words, which did not "accurately reflect the process of the James committee".
He added: "I want to make it clear that the conclusions of the James committee represent the settled view of the Conservative Party on cutting waste, removing unnecessary bureaucracies, giving taxpayers value for money and protecting and enhancing frontline public services."
The Times reportedly obtained a tape recording of the remarks, made at a private meeting of the Thatcherite Conservative Way Forward Group on Wednesday.
'Cat out of bag'
Seizing on Mr Flight's resignation, Health Secretary John Reid said he had "let the cat out of the bag" about Tory cuts.
"He is not just any old Tory MP - he is the architect of the Tory tax and spending plans over the last two years," Mr Reid told the BBC Radio 4 Today programme.
"He has worked alongside Oliver Letwin in devising the medium-term strategy and what it confirms is £35bn cuts is the minimum they are aiming for."
The Tories' shadow chief secretary to the Treasury, George Osborne, said: "I give you an absolute assurance that there is no secret agenda, there are no secret cuts being planned.
"We have published in extensive detail exactly how we propose to reduce government activity in certain areas but very substantially increase spending on health, education and policing."
Lib Dem Treasury spokesman Vince Cable said: "Howard Flight is a very senior, indeed respected, figure who was at the heart of Conservative economic policy-making.
"The problem all along has been that they have been promising simultaneously to cut taxes, improve public services, and reduce the budget deficit."
Mr Cable told Today: "They say 'We will just cut waste' and hope people will take them on trust and now they have been found out."
Mr Flight, first elected in 1997, is one of the most Eurosceptic members of the Conservative Parliamentary Party.
He has said he would never vote in favour of a single European currency.