He was found to be almost 10 times over the limit with 189mg of alcohol per 100ml of blood in his system - the legal limit for a pilot is 20mg.
Katsutoshi Jitsukawa, 42, who works for Japan Airlines, was arrested on 28 October after failing a breath test.
Tests found the 42-year-old first officer recorded 0.18 blood-alcohol reading, nearly 10 times the legal limit for a pilot and four times the amount for driving.
On Thursday, Jitsukawa pleaded guilty to exceeding the alcohol limit at Uxbridge Magistrates' Court.
However, a bus driver noticed the smell of alcohol while transporting flight crew to airplanes and other destinations and notified a relevant airport official.
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Just 50 minutes before a flight from London to Japan was scheduled to depart, the pilot was arrested for having nearly 10 times the legal amount of alcohol allowed in his system.
JAL said the flight was delayed more than an hour and had to be operated by the remaining two pilots.
Under Japan's Road Traffic Law, drivers are considered to be driving under the influence if they have an alcohol-per-liter-of-breath reading of 0.15 milligrams or higher.
The co-pilot will be sentenced on November 29, according to a United Kingdom court official.
Julian Monaghan drank three double vodkas in his hotel room on an empty stomach before he was due on board a flight from Gatwick Airport to Mauritius on January 18.
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A British Airways pilot was jailed for eight months in June after he was caught more than four times the alcohol limit on duty.
JAL's company rules prohibit its pilots from drinking alcohol less than 12 hours before flying any planes.
"We are certain (the in-house breath test) wasn't conducted properly", Muneaki Kitahara, JAL's head of communications, told a press conference.
It apologized today for the incident.
In a statement JAL said: "The Company does not condone the individual's actions, as safety remains our utmost priority".
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The plane departed London after a delay of one hour and 9 minutes.