The United States Treasury concealed $40 billion in likely taxpayer losses on the bailout of the American International Group earlier this month, when it abandoned its usual method for valuing investments, according to a report by the special inspector general for the Troubled Asset Relief Program.

“In our view, this is a significant failure in their transparency,” said Neil M. Barofsky, the inspector general, in an interview on Monday.

In early October, the Treasury issued a report predicting that the taxpayers would ultimately lose just $5 billion on their investment in A.I.G., a remarkable outcome, since the insurance company was extended $182 billion in taxpayer money in the early months of its rescue. The prediction of a modest loss, widely reported as A.I.G., the Federal Reserve and the Treasury rushed to complete an exit plan, contrasted with an earlier prediction by the Treasury that the taxpayers would lose $45 billion.

“The American people have a right for full and complete disclosure about their investment in A.I.G.,” Mr. Barofsky said, “and the U.S. government has an obligation, when they’re describing potential losses, to give complete information.”