Author Jeff Pearlman said the US President’s current outbursts may stem from enduring resentment at how, more than 30 years ago, the National Football League refused to let him join its “ridiculously exclusive club” because he was viewed as a “scumbag huckster”.

Pearlman told Newsweek that in 1984 Mr Trump, then a 40-year-old New York property tycoon, held a secret meeting with NFL commissioner Pete Rozelle and effectively offered to “do whatever it takes to join the NFL”.

Thirty-three years after the snub, Mr Trump is now attacking the NFL over some players “taking a knee” (kneeling) when the American national anthem is played before games, in protest against police brutality.

The book will not be published until next year, but in one of his blogs Pearlman appeared to give an indication of its theme and his view of the role played by Mr Trump, who in 1984 and 1985 owned the New Jersey Generals USFL team.

“ You are bigger, younger, and stronger than I, which means I’ll have no regrets whatsoever punching you right in the mouth the next time an instance occurs where you personally scorn me, or anyone else, who does not happen to salute and dance to your tune .”