The enormous increase in valuation of the largest US tech stocks is unlikely to continue, according to Rob Arnott, partner and chairman of the board of Research Affiliates, with the extreme concentration of assets in just a handful of companies flagged as a major threat to the bull market.

Facebook, Apple, Netflix, Microsoft, Amazon and Alphabet (the parent company of Google) have a combined market capitalisation of about $4.2tn, said Arnott.