Fannie Mae and Freddie Mac, the largest buyers of US home loans, on Monday agreed to new rules requiring them to buy mortgages only from banks that rely on unbiased appraisers to value homes.

The deal, brokered by Andrew Cuomo, New York attorney-general, is intended to address one possible cause of the current mortgage crisis, the alleged use by banks of “internal” or “preferred” appraisers who had incentives to inflate home values.