The valuation of US private equity house The Carlyle Group is understood to have dropped by more than a third to about $12.5bn (€9.4bn), according to documents filed by one of its largest shareholders, in a rare glimpse into the worth of privately held buyout firms.

An annual report filed by Mubadala, a sovereign wealth fund based in Abu Dhabi and one of Caryle's largest shareholders, revealed a 2bn dhiram ($544m) impairment charge on its stake in Carlyle - equivalent to 37% of the $1.35bn it originally paid for a 7.5% holding, which valued Carlyle at $20bn at the time.