U.S. home foreclosures doubled in September from a year earlier as subprime borrowers struggled to make payments on adjustable-rate mortgages, RealtyTrac Inc. said.

There were 223,538 foreclosure filings last month, including default and auction notices and bank repossessions, an 8 percent decline from August. California had the most with 51,259, and Florida was second with 33,354. The national foreclosure rate was one for every 557 households, said RealtyTrac, based in Irvine, Calif.

Foreclosures are deepening the U.S. housing recession by pushing more homes onto a market where sales and prices are dropping. There's a 10-month supply of unsold homes, the highest in at least eight years. As many as half of the 450,000 subprime borrowers whose mortgages will reset through November may lose their homes because they can't afford the higher payments, according to a report by Credit Suisse Group.

"The truth of the matter is that borrowers are going into default as soon as they hit their adjustments," said Rick Sharga, executive vice president of marketing RealtyTrac. The company sells foreclosure information and has a database of more than 1 million properties from 2,500 U.S. counties.

Prices in 20 U.S. metropolitan areas fell 3.9 percent in the 12 months through July, the most on record, according to the S&P/Case-Shiller home-price index.

Adjustable-rate mortgages to subprime borrowers, people with limited or weak credit histories, accounted for 7.3 percent of all home loans and 44 percent of all new foreclosures, according to the Mortgage Bankers Association in Washington.

The adjustable loans have lower introductory, or "teaser," rates that can re-set by 50 percent to 100 percent after two or three years, meaning that a $1,500 mortgage payment can increase to as much as $3,000 after the adjustment, Sharga said.

Foreclosures on loans made in 2005 may "start to wind down" at the end of the year, while loans made in the first half of 2006 will probably lead to additional foreclosures in the middle of 2008, Sharga said. "This wave ends in December, and another wave starts in May," he said.

Nevada had the highest foreclosure rate in September among U.S. states, one for every 185 households, and 5,504 filings, an increase of 187 percent from a year ago, RealtyTrac said. Florida's rate was the second highest at one for every 248 households, and California's was third at one for every 253 households.