Katsutoshi Jitsukawa, 42, who works for Japan Airlines, was arrested on 28 October after failing a breath test.
He was found to have 189mg of alcohol per 100ml of blood in his system - the legal limit for a pilot is 20mg.
The first officer pleaded guilty to exceeding the alcohol limit at Uxbridge Magistrates' Court on Thursday.
Japanese broadcaster NHK reported that police were alerted by the driver of a crew bus who smelled alcohol on the pilot.
He had been due to be part of a crew flying a Japan Airlines (JAL) flight JL44 to Tokyo but failed a breath test 50 minutes before the departure time.
The Boeing 777 aircraft took off after a 69-minute delay.
JAL issued an apology and pledged to "implement immediate actions to prevent any future occurrence", adding that "safety remains our utmost priority".
The drink-drive limit in England, Wales and Northern Ireland is 80mg, compared to 20mg for pilots.
Jitsukawa was remanded in custody and will be sentenced at Isleworth Crown Court on 29 November.
In June, British Airways pilot Julian Monaghan was jailed for eight months for being caught on duty with 86mg of alcohol in his system.
He had turned up for work at Gatwick Airport after drinking three double vodkas.