WASHINGTON — Fannie Mae has delayed the rollout of its new automated underwriting system that will require mortgage lenders to use trended data for the first time in submitting their loans for approval.

The Desktop Underwriter 10.0 was slated to go live June 25. But after problems arose during the testing phase, Fannie decided to push the date back until the last week of September.

"We encountered difficulties when our DU 10.0 testing environment became unstable and we had to shut it down for diagnostics and repairs," a Fannie Mae spokeswoman said Thursday in response to an inquiry.

Trended data provides 24 months of actual scheduled payments and balance data for each trade line — mortgage, credit card, utilities, etc. It is a massive amount of data compared to the current credit report, which includes a FICO score and tells the lender how many times a borrower has missed a payment in the last 12 to 24 months.

"We also heard feedback that some of our customers could use a little more time for this implementation. So we decided to postpone the release," the Fannie spokesman said. "But the delay was more a result of the difficulties we encountered in the testing environment."

Fannie announced the delay late Tuesday. It said it will now launch the program on Sept. 24.