The collective deficit of the UK's private sector final-salary pension schemes worsened in May, the Pension Protection Fund (PPF) said.
At the end of the month, the deficit of the 7,300 schemes it monitored widened to £42bn from just £2bn in April.
However, the position of the schemes is still better than a year ago, when their combined deficit stood at £179bn.
The PPF said the deterioration in the past month was due to the falling value of assets such as shares.
"Total scheme assets amounted to £895.8bn in May 2010, representing a decrease of 1.9% over the month and an increase of 15% over the year to May 2010," the PPF said.
"Meanwhile, scheme liabilities [the value of assets needed to pay pensions]... increased 2.4% over the month from £915bn in April 2010 [to £937bn]."