Fannie Mae and Freddie Mac, the US government-sponsored mortgage financiers, on Thursday warned that the market for so-called “jumbo” mortgages might freeze up again if Congress did not permanently increase the size of mortgages the two companies were allowed to buy.

A temporary increase granted under the Economic Stimulus Act in February has helped to ease punitively high rates for large mortgages in some high-cost areas, after purely private sources of finance all but dried up in the credit squeeze.