Lehman Brothers, the bust investment bank, triggered one of the biggest corporate debt defaults in history yesterday as it emerged that the US Federal Reserve is harbouring grave concerns about whether Washington’s $700 billion (£413 billion) bailout fund will avert a financial meltdown.

An auction of Lehman’s bonds yesterday determined that the bank’s borrowings were worth only 8.625 cents on the dollar. The valuation leaves the insurers of the debt a bill of about $365 billion. It is not clear whether the insurers, which are required to settle the bill in the next two weeks, will be able to pay – a development that could further undermine increasingly stressed capital markets.