The Nobel economics laureate told the Financial Times that his valuation confidence indices, based on investor surveys, showed greater fear that the market was overvalued than at any time since the peak of the dotcom bubble in 2000.

"It looks to me a bit like a bubble again with essentially a tripling of stock prices since 2009 in just six years and at the same time people losing confidence in the valuation of the market," he said.

However, he made clear that it remained impossible to time any fall in the market, and cast doubt on whether stocks would drop should the Federal Reserve raise rates later this week.

"I'm not looking for any big effect," he said. "It's been talked about for so long, everyone knows that it's coming. It's just not much of a big deal."