The pensions deficits of the 20 charities with the largest fundraising incomes rose by about £100m in the second half of 2009, according to estimates released by pensions actuary Alexander Forbes.
The estimates, based on the most recent accounts of those 20 charities, say that their total deficits rose from about £620m on 31 July 2009 to about £720m at the end of the year.
The estimates suggest that pension schemes contain only three-quarters of the assets needed to fund their liabilities, which total about £2.8bn.
It says Barnardo's, which recently launched an emergency fundraising appeal, has a predicted pensions deficit of almost two-thirds of its annual income.
Colin Mouque, director of actuarial services at Alexander Forbes, said the pensions outlook could be better for charities in 2010.
"Some of the major factors affecting pensions are bond yields, equity prices and inflation," he said.
"The ideal situation for charities would be if gilt yields and equity prices were to rise, and inflation were to stay low.
"The first two are happening at the moment, but inflation is currently rising. If we come out of recession smoothly, I would expect the pension position of charities to improve."