Billionaire investor George Soros is to tell US lawmakers on Tuesday that “a bubble in the making” is under way in oil and other commodities and that commodity indices are not a legitimate asset class for institutional investors.

He is expected to tell a congressional committee that rising oil prices are the result of a number of fundamental changes and factors in the market, but that the relatively recent ability of investment institutions to invest in the futures market through index funds is exaggerating price rises and creating an oil market bubble.