These warnings follow one of the weakest years ever seen for profit-taking by start-up investors, and come amidst predictions of an even worse period ahead. Only six companies that were backed by venture capital went public last year, the lowest numbers since the 1970s.

The other main way for start-up investors to realise gains – selling out to other companies – also hit a low point in 2008. Only 325 were sold, the lowest number since 2003, according to Dow Jones VentureSource. This lack of profitable “exits” for investors has come just as venture capital firms were hoping finally to recover from the dotcom hang-over – since many of the companies funded at the peak of the boom are now mature enough to be sold or floated publicly.

“There's been far more money paid into our industry than being returned,” said Dixon Doll, founder of Doll Capital Management and current chairman of the National Venture Capital Association. He blamed the “massive over-funding” that accompanied the dotcom boom and the generally weak state of initial public offerings since.