Claudia Webbe, the MP for Leicester East, has been found guilty of harassment, following a trial at Westminster Magistrates’ Court.
Webbe, who was elected as a Labour MP in 2019 and was a close ally of Jeremy Corbyn, threatened to use acid against her boyfriend’s female friend.
A Labour Party spokesman has called on Webbe, who now sits as an independent, to resign from the Commons.
The party spokesperson said: “The Labour Party strongly condemns Claudia Webbe’s actions and she should now resign.”
The court heard Webbe made a string of threatening calls to 59-year-old Michelle Merritt between September 1 2018 and April 26 last year because she was jealous of her friendship with Lester Thomas.
During one, Webbe called Merritt “a slag”, threatened her with acid and said she would send naked pictures and videos to her daughters, Westminster Magistrates’ Court heard.
In another call, recorded by the complainant, Webbe is heard telling the victim to “get out of my relationship” some 11 times.
Prosecutor Susannah Stevens said Webbe harassed Merritt because she was jealous of her friendship with Thomas, a consultant at Crossrail, football coach and scout for Chelsea.
Webbe, from Islington, north London, denied harassment, claiming she only made “courtesy calls” to warn Merritt not to breach coronavirus regulations by meeting Thomas.
And Paul Hynes QC, defending, read out a string of character references from figures including Corbyn, ex-shadow chancellor John McDonnell and former shadow home secretary Diane Abbott.
But Chief Magistrate Paul Goldspring found her guilty on Wednesday following a two-day trial.
“I find she was jealous of the relationship between Lester and Michelle, whatever the nature of that relationship, she certainly thought it was something she was not happy with,” he said.
He adjourned sentencing to November 4 and released Webbe on unconditional bail but warned her she could face prison.
Webbe is under pressure to resign from parliament, triggering a by-election.
A Labour source told HuffPost UK: “The real question is who would our candidate be? I think a lot of people are wondering if the previous MP or one of his family might be running?”
Webbe’s predecessor in the seat was Labour’s Keith Vaz, who stood down from parliament after being found to have offered to purchase cocaine for others during an encounter with male prostitutes.