One of the largest investment consultancies in the world has said the "vast majority" of exchange-traded funds, which now hold over $1 trillion (€679bn) in assets, are "unattractive" for long-term institutional investors.

Exchange-traded funds have boomed in popularity since the first ETF launched in 1993, thanks to their simplicity and low fees relative to actively managed funds. In July this year they exceeded $1 trillion in assets for the first time ever.