Voluntary organisations claimed £750m in Gift Aid last year, but the Charities Aid Foundation estimates that a further £700m went unclaimed, partly because of the difficulties of administering the scheme.
Cancer Research UK employs the equivalent of seven full-time workers on administering Gift Aid.
Delegates at a Gift Aid policy forum in London, staged by the Institute of Fundraising last week, said the administrative burden was one of their biggest concerns.
The event, which aimed to stimulate debate before the consultation begins in June, considered ways to make the scheme easier to run.
The consultation comes after the announcement of a 2p cut in income tax in the Budget, which CAF has claimed will mean a £70m loss for charities.
The most popular proposal was for the Government to abandon donor declaration forms in favour of allowing charities access to HMRC's global taxpayer register so that fundraisers could identify and exclude non-taxpayers from Gift Aid claims.
Among other suggestions were giving all donors a tax-free allowance to donate to charities and improving systems for collecting Gift Aid on in-kind gifts.
Mark Astarita, director of fundraising at the British Red Cross, said an equitable tax-free allowance would prevent basic-rate taxpayers from having to give more to allow charities to claim the same amount back as they do under the current rebate system. "Why is it right for charitable giving to be cheaper for higher-rate taxpayers?" he asked.
Lekha Klouda, director of the Association of Charity Shops, said improving systems for collecting Gift Aid on donated goods would be both cheap to administer and potentially lucrative.
But Cathy Pharoah, director of research consultancy Third Sector Prospect, said there was an upside to the existing rules. "One of the advantages is that financial advisers talk to major donors about their charitable giving," she said.
Lindsay Boswell, chief executive of the institute, said the consultation was a huge opportunity to increase Gift Aid income. But he warned that charities would be disadvantaged unless realistic benchmarks were established to assess the amount of Gift Aid money available.
"Otherwise, the standard answer from government is 'you're not good at collecting it, so we don't see why we should give you more'," he said. "Such benchmarks should be a priority of the research centre on giving."