BC Partners underestimated the scale of the housing market meltdown by half in the doomsday scenario the private equity group drew up last year to stress-test the worst-case outcome of its acquisition of Foxtons estate agents.

The buy-out group measured its £360m acquisition of Foxtons in May 2006 against a potential 30 per cent drop in property sales. But even this proved hugely optimistic, as sales have since fallen 50-70 per cent, according to people close to the deal.