Prominent Tory MPs joined wealthy philanthropists to attack government plans which could reduce the amount of money people give to charity by capping tax relief at £50,000.
Influential backbench MP David Davis said the idea was an "assault on the 'big society'" while Tory party treasurer Lord Fink said it might "put people off" giving money to charities.
Meanwhile, a group of 46 big donors wrote to the Sunday Telegraph saying the plans would put a "brake on philanthropy".
There was even a suggestion Prince Charles might privately lobby against the move, when Philip Spedding, director of the prince's Arts & Business organisation, said he would be against the plans.
“I find it astonishing, it suggests there is no joined up thinking in the government,” he said.
“It sends a message that giving out major gifts is somehow suspect and dodgy.
“I would be surprised if the prince didn’t share everyone’s concerns about the developments because he’s someone who cares very passionately about culture and encouraging people to give.
"I can imagine he would find it worrying."
Tory backbencher and influential environmental campaigner Zac Goldsmith wrote in the Mail on Sunday he was "ashamed of the plan".
The plans, which would cap tax relief at £50,000, were part of Budget proposals intended to increase tax revenue from wealthy members of the public, but critics say they discourage charitable giving.
Liberal Democrats were left supporting the policy this morning, with Nick Clegg saying it was "not right that some wealthy individuals can use [charity tax relief] to reduce their tax bills to close to zero, often year after year".
Former party leader Paddy Ashdown told the Andrew Marr programme: "We want people to pay their taxes, we don’t want to kill off charities.
"If everyone gave their money to charities instead of paying tax there would be no schools or hospitals."
The tax relief row is just the latest to hit the badly-received Budget since Mr Osborne delivered it last month. Pasty taxes, granny taxes and the reduction of the top rate of income tax have also hurt the government.