The two men, in their 30s, left staff at the upmarket Embassy nightspot in Mayfair with hefty tips after going head-to-head to see who could accrue the larger bar tab.
The multimillionaires, who are believed to have made their money in the oil industry, were accompanied by an entourage of around a dozen friends when they arrived at the venue.
But witnesses said they were joined by a gaggle of freeloaders as the dare escalated to incredible proportions.
There was so much alcohol - including vintage champagne - that around one-third of the bar bill ended up being poured away when the men left at 3am today.
A venue spokesman said: "Nobody could quite believe what was happening - the two clients seemed to be trying to outdo each other.
"One ordered five bottles of champagne, so the next one upped it to six, then to seven, and so on.
"It was incredible. The men didn't end up drinking that much of it themselves - it was mostly consumed by the lady friends they had met inside on the night."
It equates to more than 2,000 units of alcohol - meaning it would take the men around 10 months to each consume their own share without exceeding the recommended daily guidelines.
The bar bills included 30 magnums of Cristal Brut champagne, at £1,380 a bottle, 84 bottles on Dom Perignon, 12 bottles of Dom Perignon rose, several shots of 18-year-old Glenmorangie single malt whisky, champagne-based cocktails and bottles of Peroni lager.
The haul also featured soft drinks - 44 cans of Red Bull and 14 bottles of water.
The Embassy spokesman added: "The men needed some help drinking what they bought.
"It was absolutely monumental. These kind of bar bills are rare, the sort of thing that happens once a year - and then we had two in one night.
"Even for Mayfair, the amount they spent was extremely high."
He added: "The two men left in good spirits, they weren't legless or anything like that.
"But they obviously couldn't drink all they had bought - about one-third was not consumed."
The bill included around £19,000 in service charges, of which the staff take a percentage.
"The staff will have done very well out of them," the spokesman said.
He said the venue was happy to honour a request from the customers to keep their identities private.
The Government advises people should not regularly drink more than the guidelines of three to four units of alcohol for men a day (equivalent to a pint-and--half of 4% beer) and two to three units of alcohol for women a day (equivalent to a 175ml glass of 13% wine).
"Regularly" means drinking every day or most days of the week.