Organisations representing hundreds of Norfolk and Suffolk charities last night said the chancellor's plans to cap tax relief for donors would hit them at the worst possible time.

Groups including Voluntary Norfolk, Age UK Norfolk, Beat, Leeway and the Norfolk Community Foundation are among more than 800 which have signed up to the Give it Back George campaign to oppose the measure aimed at tax dodgers.

During his budget last month George Osborne announced plans to cap tax reliefs at �50,000 or 25pc of a person's income to stop wealthy people using them to cut their tax bills.

But the measure is set to include tax relief on charitable donations and has infuriated organisations who fear donors may now be put off giving large amounts.

Major charities including Cancer Research UK and Macmillan Cancer Care are opposing the plans but last night the plight of Norfolk's many smaller organisations was also raised.

Brian Horner, chief executive at Voluntary Norfolk, said philanthropists would often choose to set up trust funds to benefit a number of smaller groups instead of making large donations to a single cause.

He said: 'Our concern is that if those people are deterred from doing that, that means there will be less funding available for smaller voluntary groups to apply for at a time when trust funds have got less money to give out anyway.

'We have signed up to this campaign, not because it will directly benefit Voluntary Norfolk, but because it will impact on many hundreds of smaller voluntary groups in the county.'

Norfolk Community Foundation said the plans could affect 20pc of its annual income which goes on to support charitable groups across the county.

Hilary MacDonald, chief executive of Age UK Norfolk, said the plans came at the worst time for charities who were already struggling to secure funding.

She said: 'Like all charities donations are vital to our work. This cap would be unwelcome news at any time but given the economic climate and the government's vision for the Big Society, it makes absolutely no sense at all.'

Beat, a national charity supporting people with eating disorders based in Norwich, warned the Chancellor's plans could ultimately affect the increasingly in-demand services provided by the voluntary sector.

Spokesman Leanne Thorndyke said: 'Without major donations and grants we will not be able to continue to develop our services and support people affected by eating disorders.'

Last night a Treasury spokesman said it was aware of the potential impact on charities and was 'proactively exploring with philanthropists ways to ensure this new limit will not significantly impact upon charities that depend on large donations'.