Katsutoshi Jitsukawa, 42, who works for Japan Airlines, pleaded guilty to exceeding the alcohol limit at Uxbridge Magistrates' Court on Thursday.
Japanese broadcaster NHK reported that the driver of a crew bus alerted police after smelling alcohol on the pilot.
When tested for alcohol after suspicions were aroused, police found he had 189mg of alcohol per 100ml of blood in his system.
He had reportedly consumed two bottles of wine and more than 1.8 litres (nearly four United States pints) of beer over six hours on the night before the flight.
Due to the arrest, the flight's departure for Haneda, scheduled for 7 p.m. Sunday local time, was delayed by 1 hour and 9 minutes.
The legal limit for pilots is 20 mg, while drivers in England, Wales and Northern Ireland are allowed as much as 80 mg.
In June, British Airways pilot Julian Monaghan was jailed for eight months for being caught on duty with 86mg of alcohol in his system.
"It is regrettable because (the co-pilot's drinking) could have affected the safe operation of the flight and damaged public confidence in Japanese aviation", Ishii said.
The co-pilot will be sentenced on Nov 29, according to a British court official.
JAL had prohibited its pilots from drinking alcohol 12 hours before getting on their flights.
JAL subsequently apologized for the breach and admitted that an in-house breath test was not conducted properly.
Pilots should be highly responsible and should adhere to safety standards when it comes to both the craft, as well as themselves.
In a statement JAL said: "The Company does not condone the individual's actions, as safety remains our utmost priority".
His heavy drinking also forced the company to operate the flight from Heathrow airport to Haneda airport in Tokyo with two pilots rather than the normal three.
The male pilot in his 40s called in sick after a night of drinking on the remote resort island of Ishigaki in southern Okinawa prefecture, the company said.
Following the two incidents, the transport ministry urged airline companies to strictly comply with rules on drinking.