Statistics from the British Market Research Bureau show that the figure has fallen from 70 per cent in 2005 to 63 per cent this year.
The figures are based on the questionnaire responses of 25,000 randomly selected people over the age of 15 throughout the UK.
"The sample captures a lot of low-level donors and shows people are feeling the squeeze," said Cathy Pharoah, co-director of the Economic and Social Research Council's Research Centre for Charitable Giving and Philanthropy. "Given the recessionary environment, it's possible the trend will continue, and charities should certainly be preparing for it."
Kate Green, head of Think Intelligence, said charities had seen this coming. "The drop is significant, but it's no surprise within the sector," she said.
"It has been increasingly difficult to recruit new donors, so charities have shifted their focus. They have for some time been concentrating more on donor retention than on donor acquisition."
Green said it was difficult to pinpoint what was causing the drop, but that the state of the economy was an obvious underlying factor. "It is too early to say this trend is a reflection of the economy, but we do need to be aware that it could cause an additional impact later this year," she said.
The survey also showed that only 9 per cent of respondents had donated online, compared with 31 per cent by post. The most popular donation method was by cash or cheque (46 per cent). Only 3 per cent of those surveyed had donated through payroll giving.
Pharoah said planned giving was more resistant to donor fluctuations and could support charities in leaner times. "It takes more effort to change direct debits or legacies, so a shift towards more regular donor-focused fundraising would definitely help charities," she said.