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Banks are tightening mortgage-underwriting standards, even as a plunge in U.S. interest rates leads to more demand from home shoppers and existing borrowers who want to refinance.

As mortgage rates fall further below 4%, it's a great opportunity for home shoppers to take out a new loan or households with outstanding debt to refinance.

Problem is, banks are becoming more reluctant to lend, a new survey showed. 

The Mortgage Bankers Association, an association of home lenders including JPMorgan Chase (JPM - Get Report) , Bank of America (BAC - Get Report) and Wells Fargo (WFC - Get Report) , said its index of credit availability slid by 3.9% in August, the most since December 2018, reflecting a tightening of underwriting standards by banks.

The latest survey supports the contention of a growing number of economists and Wall Street analysts who have warned that falling interest rates might lead banks to cut back on lending -- especially as signs mount that U.S. growth is slowing.

"It's possible some lenders may be tightening credit in expectation of a slowdown," Joel Kan, associate vice president of economic and industry forecasting for the Mortgage Bankers Association.

A slowdown could curb jobs growth or even lead to layoffs, typically one of the biggest factors driving an increase in mortgage defaults.   

Loan applications have surged in recent months, fueled by borrower demand as the average fixed contract rate on a 30-year conventional mortgage slid to 3.82% last week, the lowest in almost three years. As recently as November, the average rate was above 5%.