More than a third of $131bn (€87.1bn) worth of investment grade-rated loans and bonds downgraded to junk or speculative grade last year were issued by companies owned by some of the largest private equity groups, according to rating agency Standard & Poor’s.

In a report yesterday, S&P said 13 out of 42 companies it downgraded to junk from investment grade rated were private equity-backed, including some of the biggest buyout targets - Canadian telecoms group BCE and US utility TXU Corporation.