Sir Alan Walters, whose role as Margaret Thatcher's personal economic adviser provoked turmoil in her government, has died at the age of 82.
His influence over the prime minister led to Nigel Lawson's resignation as chancellor and sowed the seeds of her resignation from Downing Street.
Sir Alan, who suffered from Parkinson's disease, died at home after a sudden deterioration in his health.
Baroness Thatcher said: "Alan Walters was the best economic adviser any prime minister ever had. He was a great public servant, and to me personally he was the finest of friends."
Sir Alan, who obtained his economics degree by correspondence course, argued from the early 1960s that strict monetary targets were the key to handling the economy.
He initially served as Lady Thatcher's chief economic adviser between 1981 and 1984. Sir Alan continued advising her from the US and returned in 1989 to resume his old role. Five months after his return, Lord Lawson walked out of the Cabinet amid taunts that there were two chancellors.
His departure plunged the Tory Thatcher government into a crisis from which it never properly recovered. Lady Thatcher was ousted from office just 13 months later.