Twenty-three per cent of 2,000 respondents to a survey carried out last month said they had reduced their donations since January.
The figure equates to more than 10.7 million people if the results are applied to the UK population as a whole, according to Investec.
Twenty-six per cent of respondents said they had given nothing to charity in the first six months of 2010 and a further 48 per cent have given £50 or less.
Six per cent of respondents to the survey said they had reduced their donations to charity to nothing this year, and a further 12 per cent said they had halved the value of their donations since January.
A spokesman for Investec said: "Our findings suggest that in this age of austerity charities are suffering from falling donations as people have less money."