The Tory donor, who has given millions to charities working on crime prevention, education and child protection, criticised ministers for implying that philanthropists support such organisations as a tax dodge. He is the latest senior Conservative to join the growing revolt against the decision in last month’s Budget to cap tax relief on charity donations to 25 per cent of an individual’s income or £50,000, whichever is greater.

Lord Ashcroft said giving money to charity was central to David Cameron’s flagship “Big Society” plan. “It would be a sad state of affairs – and a profoundly un-Conservative one – if we came to judge a person, company or country by how much tax we all paid,” he said.

He added: “The Government should encourage people to give, not call into question the motives of those who do so.”

Later, a Labour attempt to defeat the Budget’s cut in the 50p top tax rate on incomes over £150,000 a year to 45p was defeated by 256 votes to 323, a government majority of 67.

As pensioners lobby Parliament today, Labour will force a Commons vote over the Budget’s freeze on age-related tax allowances. Ed Balls, the shadow Chancellor, released research by the Commons Library showing that “well over half” of pensioners affected by the “granny tax” will have an income well below the average taxpayer, contradicting the Government’s claims that wealthy pensioners will bear the burden of the change.