The Federal Housing Administration, which has insured record numbers of home mortgages in recent months as low interest rates touched off an avalanche of home buying and refinancing, ran out of authority to continue guaranteeing loans at midnight last nightamid predictions Congress might not be as swift to extend the agency's operating ability as in the past.

FHA also is nearing the $74.2 billion limit on the volume of loans it can insure. Once the agency gets new operating authority, it will use up the credit authority in about a week, a Department of Housing and Urban Development spokesman said.

Congress, which has approved five temporary extensions of the FHA's operating authority for the fiscal year ending Sept. 30, has included increases in the credit limit and authorization extensions in housing and supplemental appropriations bills now under consideration in the Senate and House. The housing bills would extend operating authority through fiscal 1987, ending in September 1987. None of these measures is expected to be acted on within the next few days, a Senate Appropriations Committee aide said.

The appropriations measure, which was being considered by the Senate yesterday, would raise the FHA credit limit to $132 billion this year.

In recent months, both houses of Congress passed temporary extensions of operating and credit authority quickly, before any delay in loan processing occurred, but similar action might be more difficult now, the aide said. A House staff aide said the temporary extension could get quick approval in that body after it acts on the housing authorization bill sometime next week.

Some senators and representatives, however, would prefer to wait until a full housing bill or the supplemental appropriations legislation can be passed. Others have said they might attach riders to a temporary extension, including one to change the selection criteria for awarding Urban Development Action Grants, which would cut deeply into House support for the extension bill.

FHA field offices will continue to accept and process loan applications as they have done during previous expirations of authority, a Department of Housing and Urban Development spokesman said.

If the credit limit is reached without an extension, the FHA will stop accepting applications, an action that would hold up 10,000 mortgages a day, according to Jane DeMarines, of the Mortgage Bankers Association of America.

Although business has slowed slightly in recent weeks, 120,000 FHA-insured mortgages were issued during the first half of May, four times more than would have been approved in an average normal month before the precipitous drop of interest rates began, DeMarines said.