Oxfam suffered a 15 per cent fall in the amount of goods donated to its shops last year. It said that despite figures showing a rise in retail sales, donations of clothes, books, toys and crockery continued to be down during January.
The group said donations to its network of 700 shops acted as a barometer to retail trends in the UK, rising and falling in line with the strength of the high street.
It warned that despite recent figures showing a rise in retail sales, donations continued to be down during January, suggesting the recession was not yet over.
More than 80 per cent of Oxfam's total income from its shops comes from the sale of goods that are donated by the public.
David McCullough, Oxfam's director of trading, said: "This is a crucial moment for us. Over the last 18 months, people have been buying less, replacing less and therefore donating less to Oxfam.
"Now sales are apparently on the increase, we can only wait to see if this is reflected by an increase in donations to our shops.
"We rely on the generosity of the public for their stock. Without continued donations of everything from clothing to cookware, Oxfam shops could not continue to play such a key role in communities across the UK."
But the group said it had seen an increase in purchases from its shops, with sales up 5 per cent year-on-year.
Book sales rose by 7 per cent during 2009, which it attributed to its nationwide book festival Bookfest, while music sales also rose, partly due to a partnership with the Arctic Monkeys, under which it sold exclusive formats of the last two singles from the band.
Mr McCullough said: "The evidence we're seeing through our network of 700 shops from Inverness to Penzance makes us sceptical of reports of a boom in the high street.
"Discretionary spending remains low, big sales are starting earlier and discounting harder, and we're having to work harder than ever to maintain the high quality of donations."
But the group said despite the 15 per cent fall in donations that it has suffered, it was faring better than many other charities due to a range of measures it had taken to help it weather the credit crunch.
These include becoming the first major charity shop to sell things online, with internet sales expected to contribute nearly £2.4 million during the year to the end of March.
It has also increased the number of donation banks across the UK by around 10 per cent and set up the Clothes Exchange with Marks & Spencer, under which people receive a £5 M&S shopping voucher in exchange for donating clothes to Oxfam.