New delinquencies on reworked mortgages held by bonds without government backing jumped in September, a sign that some of the fuel for housing’s recovery isn’t sustainable, according to JPMorgan Chase & Co.

A record of more than 28,000 modified home loans within so- called non-agency securities turned delinquent, a rise of 24 percent from the prior month, JPMorgan analysts said in a Nov. 16 report. The share of all non-agency loans between 30 and 60 days past due soared 0.44 percentage point to 3.54 percent, the highest since February 2010, data compiled by Bloomberg show.

Aggressive modification activity has partly driven the sharp decrease in the housing market’s “shadow inventory” tied to bad loans, the analysts led by John Sim wrote. While individual homeowners with reworked mortgages are performing better, an increase in the total number who’ve gotten aid means they represent a threat to rising property prices.

“We are now seeing a wave of re-defaults from the modifications over the last two years that failed,” the JPMorgan analysts said. “This wave should last through 2013.”

Shadow inventory in the U.S. fell to 2.3 million homes as of July, down 10.2 percent from a year earlier, according to a report last month from CoreLogic Inc. The firm’s tally includes seriously delinquent loans, homes in foreclosure and bank-owned properties that haven’t been listed.

After a 35 percent drop from a July 2006 peak, home prices in 20 large metropolitan areas gained 8.8 percent from February through August, according to an S&P/Case-Shiller index. Sales of previously owned properties climbed 2.1 percent in October to a 4.79 million annual rate, exceeding economists’ forecasts, figures from the National Association of Realtors showed today.

Seasonal Trends

Seasonal trends and fewer business days on which borrowers could make payments at least partly explain the September surge in early delinquencies among non-agency loans, analysts at financial institutions including Credit Suisse Group AG and Amherst Securities Group LP said in reports. The data were released mostly on Oct. 25.

A jump in first-time delinquencies seemed largely contained to loans serviced by Bank of America Corp., according to the analysts from Amherst, and those at JPMorgan, who wrote that “it may be possible there were reporting lags or other operational problems” that “artificially” boosted the lender’s figures.

While modified loans are increasingly important to non- agency investors, there’s “significant uncertainty” about their future performance “given the lack of historical precedent for the current situation,” Nomura Securities International analysts including Paul Nikodem wrote in a Nov. 16 report.

Subprime Loans

About 35 percent of outstanding securitized subprime loans have been modified and more than 25 percent of so-called option adjustable-rate mortgages, according to the report. About $216 billion of securitized non-agency mortgages are being paid on time after previous delinquencies, Amherst data show.

Recidivism rates after 12 months for modified subprime mortgages have declined to about 40 percent from almost 80 percent for loans reworked in the third quarter of 2008, reflecting loan servicers offering larger payment reductions and more cuts to balances, according to the Nomura analysts.

Fannie Mae and Freddie Mac’s burgeoning holdings of modified mortgages, which drove non-performing loans at the government-supported companies to a record last quarter, also cast “doubts on the true health of the housing recovery,” Jim Vogel, an FTN Financial analyst, wrote in a Nov. 16 report.

‘Thorny Issues’

The firms owned $195 billion of restructured loans on which they were accruing interest as of Sept. 30, according to the report. The debt is also on the “list of thorny issues” that must be addressed before the companies can be replaced, he said.

Values of securities in the almost $1 trillion non-agency market are little changed this month even after the release of the delinquency data and amid slumps in assets including stocks and high-yield company bonds.

Typical prices for the senior-most bonds backed by option ARMs were unchanged last week at 62.8 cents on the dollar, up from 51 cents at the start of 2012, Barclays Plc data show. Option ARMs can allow borrowers to pay less than the interest owed by increasing their balances.