Today he appeared at Crawley Magistrates Court where he pleaded guilty to being over the limit while reporting for flying duty.
Wearing a dark suit and tie, Monaghan, who divides his time between South Africa and Harmondsworth, west London, spoke only to confirm his name, date of birth and address as being under the care of his solicitors.
Mel Wotton, prosecuting, said the incident happened at around 8pm at Gatwick Airport on January 18 this year.
Monaghan, the first officer, had boarded the flight along with the pilot and was preparing to start the plane.
She said: 'One of the aircraft technicians was concerned they could smell alcohol on the pilot.
'Bosses spoke to police who duly arrived and they could also smell alcohol on his breath.'
She said a breath test was carried out and he was found to have 52mg of alcohol in his breath - the legal limit is 9mg for a pilot.
The court heard a blood sample was subsequently taken and found it have 86mg in 100ml of blood - the drink drive limit is 80mg but the legal limit for a pilot is just 20mg.
Defence lawyer Emlyn Jones said Monaghan had only had one vodka and coke more than eight hours before the flight was due to take off.
He said pilots had to follow a policy of an eight-hour alcohol prohibition before any flight which Monaghan had observed.
Mr Jones said: 'His prohibition began more than eight hours before hand. However he could not sleep and spent he day without the food and without drink.'
'He was appalled, astonished and horrified when he found he was over the limit.'
Mr Jones said Monaghan was not due to take up responsibility of take off and would not have flown the plane until they were three hours into the flight.
He said that although the matter was 'very serious' the level of alcohol was so low that if it was a drink drive case then he would not have been charged.
Mr Jones said: 'No harm was caused, His culpability was somewhat lower because he was careless rather than wilfully ignoring a prohibition of alcohol before the flight.
'By his guilty plea he has thrown away the only career he has every known. He has been a pilot all his life.
'By his own misconduct he has suffered. He has been punished enough.'
Adjourning the case for sentence at crown court next week, Chairman of the Bench Dr David Wiggins, warned him he faced an immediate custodial sentence
Dr Wiggins said: 'The comparison with drink driving is somewhat spurious as flying 777 is a bit different to driving a car.'
Monaghan joined British Airways, whose motto is 'To fly, to serve', in 2001. First officers at the airline typically earn more than £60,000.
A British Airways spokesman said: 'This behaviour is completely unacceptable and not what we expect from our highly professional fleet of pilots.
'The safety and security of our customers and colleagues is always our top priority.'
She added: 'Julian Monaghan no longer works for the airline.' Monaghan will appear to be sentenced at Lewes Crown Court on Tuesday June 12.
What is the alcohol limit for pilots?
Pilots are subject to much stricter alcohol restrictions than drivers.
The legal alcohol limit for pilots is 9mg per 100ml of breath, but for a driver in Britain it is 35mg per 100ml.
The restriction on alcohol in the bloodstream is 20mg per 100ml for pilots and 80mg for drivers.
