Warren Buffett has hit out at rising US opposition to investments by sovereign wealth funds, saying their recent buying spree was a function of misguided trade policies and not "some nefarious plot by foreign governments".

In his closely-watched annual letter to shareholders, the 77-year-old billionaire investor revealed he had chosen four unnamed candidates to replace him as chief investment officer of Berkshire Hathaway, his conglomerate, when he dies or steps down. Berkshire has a $140bn pool of investments.