Pc Roberto-Cristian Varvara, 27, took his girlfriend, Special Constable Alexandra Chiriac, 21, on a three-day break to visit his parents in mid-October.
The officers then returned to work, ignoring the mandatory two-week period of self-isolation for international travellers.
A colleague spotted their Instagram holiday snaps, sparking a confrontation at Colindale police station.
Varvara and Chiriac have now been convicted of breaking quarantine laws and ordered to each pay £2000 fines.
Pc Lesley Cook told Westminster magistrates’ court how Varvara had discussed the trip in early October, apparently unaware that Romania had been added to the Government’s quarantine list. She said the Pc “shrugged” when warned he could not take the holiday, and he also “said he may just go to Romania and not tell anyone”. Pc Cook said she saw the pictures on Instagram on October 31, including photos of Varvara with his Romanian parents.
“I had seen Pc Varvara at work so therefore believed he had not quarantined,” she said in a statement to the court.
Pc Cook said Varvara initially denied taking the trip and was evasive.
“I was getting annoyed with PC Varvara and said to him: ‘Don’t f***ing lie to me, did you go to Romania?’ He was very abrupt and said ‘I don’t want to answer that question’.”
She said the officer claimed they had taken Covid tests before flying, and he believed quarantine was not necessary if the trip was less than 72-hours long.
The two officers were sent home until November 4, when their quarantine period was supposed to end. Sergeant Emily Joshi told the court Chiriac had apologised and said they believed they were exempt from the rules.
Varvara, from Northampton, and Chiriac, from Pinner, did not pay the £1,000 fines they were handed in January, sparking a prosecution. They did not enter pleas to the charge of failing to self-isolate on their return.
Both officers were ordered to pay a £2,000 fine each, plus £290 in court costs and fees, in a behind-closed-doors prosecution that concluded on March 31.
Scotland Yard confirmed one of its officers and a special constable “are subject to on ongoing misconduct investigation”.
Varvara and Chiriac’s case was among a batch of Covid-19 prosecutions that were kept secret from the public, but which have been uncovered by the Evening Standard.
Charles Oladipu, 52, was caught failing to isolate at his Hornchurch home in December last year after returning from a trip to Nigeria.
A police officer discovered he had been away from his home for up to three hours, with Oladipu claiming his absence was due to a trip to the local shop to buy an apple.
He failed to pay a £200 fixed penalty, blaming a lack of money after being furloughed from his job, and was eventually fined £1000 after pleading guilty to the quarantine breach.
Aldi Burgu, 30, was fined £1173 by the court after he was reported to police by a neighbour for failing to quarantine properly.
He was spotted receiving guests at his Morden home within two weeks of returning from Albania last November. Burgu admitted the offence in writing, having told police he had a workman round to fix his gas.
A collection of legal groups including Fair Trials and Big Brother Watch have written to the Justice Secretary Robert Buckland urging him to halt Covid-19 prosecutions using the Single Justice Procedure over concerns about transparency, as well as fears that defendants are being convicted – sometimes wrongly - of complex offences in their absence and without having access to legal advice.