Public donations to charity fell by 20% last year, receiving a total £1.7 billion less, according to a survey.
As recession forced households to cut back, both the number of people giving to charity and the amount donated declined over the past year.
The Charities Aid Foundation (CAF) and National Council for Voluntary Organisations (NCVO), who conducted the survey 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.
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.
Medical research, hospitals and hospices and children and young people were the most popular causes among donors, but religious causes received the largest average donations.
CAF chief executive John Low said: "
"We hope the fall in giving shown in our survey is a temporary decline and not the start of a damaging trend. If donations continue to fall, many charities will face profound difficulties carrying on their work and the people and communities they serve will suffer.
NCVO chief executive Sir Stuart Etherington said he was very worried by the data, adding: "I know people want to help when they can. If I could ask one thing, I'd ask people to commit to regular donations through direct debit, and give using Gift Aid, so charities can plan properly. We cannot afford to lose the services charities provide."
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.
"The government gives significant support through the tax system to encourage giving - over £1 billion went to charities last year through Gift Aid.
"We are also introducing a new Gift Aid Small Donations Scheme, which will deliver an extra £100 million a year for charities.
And we are making it easier for charities to claim Gift Aid, and will shortly bring forward proposals for the reform of payroll giving to encourage more people to give out of their salaries."