Ben Bernanke, the Federal Reserve chairman, on Friday cast doubt on whether Fannie Mae and Freddie Mac could ever return to the hybrid private-public structure they enjoyed before being seized by the government in September, saying “alternative forms” should be considered for the two home loan companies.

Speaking at a symposium on the mortgage crisis in Berkeley, California, Mr Bernanke said: “Considering some alternative forms for the government-sponsored enterprises – or for mortgage securitisation generally – during this time out . . . seems worthwhile”.