Mike Ashley has denied promising £15m to an investment banker to help him treble Sports Direct's share price during a night of "banter and bravado".

Investment banker Jeffrey Blue is suing the tycoon, alleging Ashley did not hold up his end of the bargain on an agreement between the pair. The issue stems from a conversation between Ashley and Blue, which allegedly took place in the Horse & Groom, a London pub, in 2013. The banker maintains the tycoon promised to pay him £15m ($19.5m) if Blue used his expertise to double Sports Direct stock price to £8 per share within three years.

However, in a witness statement made public as the Newcastle United owner took the stand to defend himself against the claims, Ashley dismissed the allegations.

The Sports Direct founder, whose fortune is worth in excess of £2bn, told a High Court in London that it would be "ludicrous" for Blue to take seriously what he described as pub banter.

Ashley told the court the meeting with Blue in the Horse & Groom had been organised by the investment banker himself, as a way to sell Sports Direct to Esprito Santo Investment Bank (ESIB) as a potential client.

"It was a fun night, as it was intended to be, and everyone was on good form," the tycoon said on Wednesday.

"We got on well, so much so, that we ended up going out after the pub to a bar in the West End where we stayed until the early morning. I had drunk so much that I can't remember which bar we went to."

Ashley went to say he performed the role Blue had "asked of me" as per his game plan. He added during a night filled with "banter and bravado" he had also offered broker Simon McEvoy a job as Sport Direct or at Newcastle United.

"It was a drink-fuelled, fun night and it would have been obvious to Blue and anyone else in that context that whatever conversations took place involving a payment for Blue were not serious," the court was told.

"Blue is an experienced businessman and lawyer and it is ridiculous for him to suggest otherwise. I can't believe that he is now trying to take me for £14m off the back of some drunk banter that he is seeking to engineer into something more."

Ashley's version of events was partly backed by Peter Tracey, an ESIB trader who attended the event and told the court the evening was "a bit surreal".

"It was five guys and a barman in the pub, I wish you could have been there, it was a great night," Tracey told the court.

"It was a billionaire owner of a football club standing there talking about life, it was fascinating, it was great."

When the trial began on Monday, Blue stated he had received £1m from Ashley, instead of the £15m he was allegedly promised. The court was also told Ashley's business practices contravened "business orthodoxy".

The tycoon allegedly lay under tables to take a nap during work meetings he found boring and played "spoof", a drinking game, to decide who would pay for an investment bank's fees.

Ashley, aged 52, who remains Sports Direct's largest shareholder, refuted the claims, branding them "extremely vague".

The Newcastle United owner is also alleged to have vomited into a fireplace after a Sports Direct management meeting, during which he reportedly downed 12 pints and vodka chasers.

The claim states Sports Direct's directors held a senior management meeting once a week at The Lion Hotel in Worksop, a short drive away from the firm's headquarters in Shirebrook, Derbyshire.

"These meetings were like no other senior management meeting I had ever attended in all my years of investment banking experience," Blue told the court.

"On one such evening, in front of his senior management team, Ashley challenged a young Polish analyst in my team, Pawel Pawlowski, to a drinking competition.

"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. Ashley then vomited into the fireplace located in the centre of the bar, to huge applause from his senior management team."