PC Maurice Maison was late for a study session before his last exam to become a police officer when he was pulled over at Junction 2 of the M1.
He was sacked on Tuesday over the “completely reckless” incident, after a misconduct panel found he had breached professional behaviour standards.
Willesden magistrates heard the 34-year-old was stopped by PC Anthony Frankland on September 25 last year, when his Seat Leon car was clocked at 122mph - more than 50mph over the 70mph limit.
In a statement to the court, Maison said: “Firstly I would like to apologize for the manner of my driving and do accept responsibility.
“On the day in question I was running late and was attending Hendon Police college coming from where I live in Milton Keynes. I was on my last day before my final exam and did not want to miss the study session.”
PC Frankland said he encountered Maison at Wood Green police station around two months after the incident, when the rookie officer appeared to believe he was not being prosecuted.
“Maison was very polite and courteous, asking if I remembered him”, said the PC.
“I confirmed with Maison that I had remembered him and asked how he was getting on.
“Maison stated that he was getting on well and thanked me for not progressing with the speeding ticket.
“I did not comment on this and continued the conversation where I wished Maison the best of luck with things.”
PC Frankland said he later checked the computer system and confirmed that Maison was in fact due to be prosecuted.
He said the trainee officer was “very apologetic” during the initial roadside stop and had been immediately “worried this would affect his job”.
Maison, who was attached to the Met’s north area command unit, pleaded guilty to speeding but has asked the court not to issue him with a ban.
In his plea for mercy, the father-of-three said the cost of travelling to his police job by public transport would be too expensive to make it a viable career.
Magistrates adjourned the sentencing hearing until October.
Commenting after Maison had been dismissed without notice, for gross misconduct, Detective Chief Superintendent Treena Fleming said: “PC Maison’s behaviour was completely reckless and put other road users at significant risk.
“Police officers are never above the law and there was no valid reason for PC Maison to be driving at such high speeds.
“We expect all of our officers to uphold our high standards of behaviour and do not accept conduct of this nature which undermines confidence in our organisation.”