Newcastle United owner and sportswear firm boss Mike Ashley once vomited into a fireplace after a senior management meeting which was "effectively a pub lock-in", a finance expert embroiled in a dispute with the businessman has told a High Court judge.
Investment banker Jeffrey Blue said the Sports Direct boss would also nap under tables at boring meetings.
Mr Blue has sued Mr Ashley after claiming he did not stick to a commercial agreement.
Mr Ashley disputes the claim.
Mr Justice Leggatt began analysing evidence at a High Court trial in London on Monday.
The judge had heard the dispute between Mr Blue and Mr Ashley related to an alleged conversation in a London pub in 2013.
Mr Blue says Mr Ashley made a promise during a meeting in a pub called the Horse & Groom.
He says Mr Ashley promised to pay him £15 million if he used his expertise to increase Sports Direct's share price to £8 a share.
He says Mr Ashley paid only £1 million.
Mr Ashley's lawyers say Mr Blue's claim is "remarkable" and an "opportunistic try on".
Mr Blue told the judge, in a written witness statement, how he had been a frequent visitor to Sports Direct's head office in Shirebrook, Derbyshire.
He said he had attended several senior management meetings at the Green Dragon pub in Alfreton, Derbyshire.
"These meetings were like no other senior management meeting I had ever attended," he said.
"It was effectively a 'pub lock-in' with alcohol continuing to be served well beyond closing hours and fish and chips or kebabs being provided throughout the evening.
"On one such evening, in front of his senior management team, Mr Ashley challenged a young Polish analyst in my team, Pawel Pawlowski, to a drinking competition.
"Mr Ashley and Pawel would drink pints of lager, with vodka 'chasers' between each pint, and the first to leave the bar area for whatever reason was declared the loser.
"After approximately 12 pints and chasers Pawel apologised profusely and had to excuse himself.
"Mr Ashley then vomited into the fireplace located in the centre of the bar, to huge applause from his senior management team."
Mr Blue also told of Sports Direct senior management meetings in the Lion Hotel in Worksop, Nottinghamshire.
"Meetings typically commenced in the bar area of the Lion Hotel at around 8pm, with Mr Ashley drinking at the bar while in discussions with various members of the Sports Direct senior management team," he said.
"Mr Ashley would frequently end up taking important Sports Direct related business decisions throughout the evening."
Mr Blue said he first met Mr Ashley when working for investment bank Merrill Lynch in 2006.
"Mr Ashley was like no other client that anyone at Merrill Lynch had ever come across," added Mr Blue.
"By way of example, his ability to express boredom and frustration during client meetings knew no limits, including various episodes where he would lie underneath meeting room tables to 'have a nap'."
Mr Ashley, who is at the High Court hearing but not expected to give evidence until later in the week, disputes Mr Blue's claim.
David Cavender QC, who leads Mr Ashley's legal team, told the judge that Mr Blue's claim was an "opportunistic try on".
He said, in a written outline of Mr Ashley's case, that there was "contemporaneous evidence of the purported agreement".
Mr Cavender said Mr Blue's case was that Mr Ashley had made an agreement "over a heavy drinking session at a pub". He said Mr Blue's claim was "remarkable". The hearing continues.
Mr Blue says he wants £14 million damages.