WASHINGTON—The Pension Benefit Guaranty Corp.'s deficit in fiscal 2010 rose slightly to $23 billion from $22 billion the prior year, near its all-time high.

The fiscal 2010 deficit in the PBGC's insurance program for single-employer plans climbed to $21.6 billion, up from $21.1 billion in fiscal 2009. The deficit in the agency's insurance program covering multiemployer pension plans climbed to $1.4 billion, up from $869 million.

In fiscal 2009, the agency was hammered by several large losses, including its second-biggest ever: its takeover of massively underfunded pension plans sponsored by bankrupt auto parts manufacturer Delphi Corp., which the PBGC estimates will cost nearly $6.3 billion.

By contrast, the single biggest loss the PBGC incurred in fiscal 2010 was its September takeover of a pension plan sponsored by St. Vincent Catholic Medical Centers in New York.

The plan sponsored by the health care system, which filed for bankruptcy in April and shut down in May, had $267 million in unfunded guaranteed benefits, the PBGC said.

Still, the PBGC's fiscal 2010 deficit is just shy of its all-time high $23.5 billion deficit set in fiscal 2004.

“In part, this financial position is the result of inadequate plan funding and misfortunes that have befallen plan sponsors. In part, it is a result of the fact that the premiums PBGC charges are insufficient to pay for all the benefits that PBGC insures and other factors,” PBGC Director Joshua Gotbaum said in a statement.

While the PBGC has more than enough funds to pay guaranteed benefits, “we cannot ignore PBGC's future financial condition any more than we would that of the pension plans we insure,” Mr. Gotbaum added.

In addition, the agency could be hit with more big losses. It says its potential exposure to future losses from financially weak companies was about $170 billion in fiscal 2010, which ended Sept. 30, up from $168 billion the prior year.

During fiscal 2010, the PBGC took over 147 plans from financially ailing or failed employers, up from 144 the prior year. Since 1974, the PBGC has taken over 4,150 plans. In fiscal 2010, the agency paid $5.67 billion to participants in failed single-employer plans, up from $4.48 billion in 2009.