Donald Trump has likely won none of the 18 golf club championships in he claims to have been victorious, a new book reveals.

Golfers, caddies and celebrities have all said the US president cheats at golf, according to Rick Reilly, a former Sports Illustrated journalist who has played with Mr Trump and has interviewed dozens of people familiar with his alleged sporting misdeeds.

But in Commander in Cheat: How Golf Explains Trump, an extract of which was published in The Guardian, Mr Reilly reveals how the president’s cheating does not just on the course, it is off it too.

“Donald’s Trump’s boast about winning 18 club championships is a lie that’s so over-the-top Crazytown it loses all credibility among golfers the second it’s out of his mouth,” Mr Reilly wrote.

It emerged 12 of the 18 championships Mr Trump claims to have won – all at courses he happens to own – were in fact senior or super senior club championships, tournaments that restrict the younger, usually better, players from competing, Mr Reilly claimed.

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A Washington Post report from 2017 appeared to corroborate that suggestion.

Even some of the senior tournament wins may be in doubt: Mr Reilly spoke to a source at Mr Trump’s Bedminster course who told him the billionaire once won a senior championship without being present.

“He’d declared that the club should start having senior club championships for those 50 and up, but he forgot that one of the best players at the club had just turned 50,” Mr Reilly wrote.

“Having zero chance at beating the guy, he went up to his Trump Philadelphia course on the day of the tournament and played with a friend there.”

But, the source said, Mr Trump later called his own Bedminster golf shop and announced he had shot a 73 in Philadelphia and should be declared the winner.

The shop worker, wanting to remain employed, agreed and Mr Trump’s name went on the plaque. His caddie would later reportedly say Mr Trump shot “maybe 82. And that might be generous”.

Of the six remaining tournaments, three were at Trump Westchester: In 2001, when the club had yet to officially open; in 2002, when it only had an illegitimate nine holes; and in 2004.

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Of the final Westchester “win”, the course’s former executive Ian Gillule told Mr Reilly: “Well, no, I know for a fact that’s not true. He never won any in the eight years I worked there.”

The three remaining competitions Mr Trump claims were all at Trump International in West Palm Beach, which features plaques proclaiming the president’s victories in 1999, 2001 and 2009.

Hope Hicks, Mr Trump’s former spokesperson, admitted to the Washington Post in 2016 the first victory was actually a round he played with “a group of early members” before the course had opened.

Of the final two, Mr Reilly said he has “never seen a signed scorecard or spoken to any objective person who remembers him winning or not winning”.