Public donations to charity fell by 20% in real terms last year, with good causes receiving £1.7 billion less, according to a survey.
The survey for the Charities Aid Foundation (CAF) and National Council for Voluntary Organisations (NCVO) found that the number of individuals giving to charity fell in 2011/12 and amounts donated also declined, from £11 to £10 a month.
CAF and NCVO warned that charities were facing a "deeply worrying" financial situation, with some forced to cut back frontline services and make staff redundant or even facing closure because of the fall in income.
They launched a Back Britain's Charities campaign, urging individuals and businesses to give what they can and calling on the Government to modernise Gift Aid and to ensure that public bodies do not disproportionately cut funding for charities when seeking budget savings.
Data from 3,000 people, collected by the Office for National Statistics, suggested that total giving to charities by members of the public in the UK fell from £11 billion to £9.3 billion in 2011/12 - the largest one-year decline in the Survey of Individual Giving's eight-year history.
When inflation is taken into account, the £1.7 billion reduction is the equivalent of a £2.3 billion fall in donations - more than 20% of total UK giving by individuals.
The survey found that 28.4 million people gave to charity during 2011/12 - more than half of all UK adults. But the proportion of people donating to charitable causes in a typical month fell from 58% to 55%. A larger proportion of women (58%) than men (52%) gave to charity.
CAF chief executive John Low said: "The drop in giving shown by our survey is deeply worrying for those charities which rely on donations to provide vital frontline services. Combined with public spending cuts this represents a potentially severe blow for many charities. We hope the fall in giving shown in our survey is a temporary decline and not the start of a damaging trend."
A Government spokesman said: "Britain is a generous country and despite the recent economic difficulties people have continued to give both their money and time to support charities.
"It's too early to tell if these findings are a trend, but we are working hard to stimulate giving through initiatives such as new tax incentives, committing £50,000 match-funding to encourage people to support local charities and our £100 million transition fund, which is helping them to adapt to the new economic environment."