Nearly 70 percent of the Pentagon’s 96 largest weapons programs were over budget last year, for a combined total of $296 billion more than the original estimates, a Congressional auditing agency reported Monday.

The findings, compiled by the Government Accountability Office, seemed likely to add to the pressure on officials to make sizable cuts in the most troubled programs as they work out the details of a proposed $664 billion defense budget for fiscal 2010.

President Obama has said that the “days of giving defense contractors a blank check are over.” Pentagon officials have said they will finish putting together a list of proposed cuts in April.

In a letter to Congress, Gene L. Dodaro, the acting comptroller general for the G.A.O., an auditing agency, said that while there had been modest improvements in the last year, the Pentagon’s management of the contracts remained poor, and cost overruns were “still staggering.”