Banks continued to tighten lending standards for mortgage borrowers over the past three months, the Federal Reserve reported Monday; in particular, prime borrowers are now beginning to feel the credit squeeze, although banks said lending standards had tightened across the board for mortgages in all credit and lending categories. The Fed’s quarter survey of loan officers found evidence of broad credit tightening outside of just mortgages; almost no banks had eased credit terms on any type of major lending during the first three months of 2008. Market experts point to tightened credit standards as one driving reason why so many troubled mortgage borrowers find themselves unable to refinance into a new mortgage that might keep them in their home. 60 percent of U.S. banks said they had tightened standards for prime mortgages during the first quarter, up from fourth quarter numbers; among non-traditional mortgage lenders, including Alt-A, 75 percent said they had tightened lending standards. The moves by U.S. banks come as Alt-A mortgage performance has deteriorated rapidly, and cracks are beginning to emerge even among prime jumbo borrowers as well. Last week, rating agency Standard & Poor’s cut 184 prime jumbo RMBS classes, and warned on hundreds more, as it said that delinquencies among prime jumbo borrowers had increased by more than 68 percent since December of last year. HELOCs — among the most problematic class of mortgage-related lending — saw credit standards tightened 70 percent of banks in the first quarter, while 50 percent said they had tightened terms on existing secured lines of credit.