The share of borrowers who are falling seriously behind on loans backed by the Federal Housing Administration jumped by more than a third in the past year, foreshadowing a crush of foreclosures that could further buffet an agency vital to the housing market's recovery.

About 9.1 percent of FHA borrowers had missed at least three payments as of December, up from 6.5 percent a year ago, the agency's figures show.

Although the FHA's default rate has been climbing for months and eating into the agency's cash, the latest figures show that the FHA's woes are getting worse even as the housing market shows signs of improvement. The problems are rooted in FHA mortgages made in 2007 and 2008. Those loans are now maturing into their worst years because failures most often occur two to three years after a mortgage is made.

If the trend continues and the FHA's cash reserves are exhausted, the federal government would automatically use taxpayer money to cover the losses -- a first for the agency, which has always used the fees it charges borrowers to pay for its losses.