The mainstream private debt market is showing “signs of material deterioration” and the prospect of lower returns to investors, according to one of the world’s biggest investment consultancies — which is advising its big-money clients to invest elsewhere.

Willis Towers Watson, advisers to pension funds and other investors worth $2.3tn, issued the stark warning as cash continues to flood into private debt funds. Investors have committed $44bn to them so far in 2018, according to data providers Preqin.