London's Metropolitan Police said tests revealed he had 189 mg of alcohol per 100 ml of blood in his system - nearly ten times the 20 mg limit for a pilot.
JAL said the pilot is suspected of improperly clearing the alcohol check, as the airline's breath-testing equipment did not detect a problem.
Jitsukawa had nearly 189 mg of alcohol per 100 ml of blood in his system, which is nearly 10 times the permissible 20 mg for a pilot.
London's Metropolitan Police force say Katsutoshi Jitsukawa appeared at Uxbridge Magistrates Court in west London on Thursday and admitted to exceeding the alcohol limit.
A spokesman for the London police said a test on the co-pilot taken 50 minutes before the flight's scheduled departure revealed 189 milligrams of alcohol per 100 millilitres of blood in his system - nearly 10 times the 20-milligram limit for a pilot.
JAL said the flight was delayed more than one hour and had to be operated by the remaining two pilots.
The limit for drivers in Britain is 80mg.
He was ordered detained until he is sentenced on November 29.
And not just drunk, he was 10 times over the permissible alcohol limit for pilots.
As a result of the incident, the flight departed an hour late and flew with two pilots rather than three.
"We are certain (the in-house breath test) wasn't conducted properly", Muneaki Kitahara, Japan Airlines' head of communications, told a press conference.
In a statement JAL said: "The Company does not condone the individual's actions, as safety remains our utmost priority".
The incident comes after a pilot for domestic Japanese airline All Nippon Airways was too hungover to work.
Under company rules, pilots are prohibited from drinking alcohol within 12 hours before a flight.
The Transport Ministry has been requiring airlines to take measures to prevent alcohol-related incidents.
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