The MP Claudia Webbe is facing a potential jail sentence and demands to stand down from her seat after being found guilty of a campaign of harassment including threatening an acid attack.

Webbe, the independent MP for Leicester East, who was elected as a Labour candidate in 2019 but had the party whip withdrawn, was accused of harassing Michelle Merritt, a friend of her partner, with threatening phone calls. The trial was told she had called Merritt a slag, threatened to “use acid” and said she would distribute naked pictures of Merritt to her family.

Following the guilty verdict and a warning from the judge that she could face a prison sentence, Labour called for Webbe to quit her seat, which would trigger a byelection. Webbe protested her innocence and made clear that she planned to continue as an MP.

“While I’m preparing for the appeal I want to assure the people of Leicester East that I will continue to stand up for them in parliament, fighting on their behalf,” she said.

Webbe, a close ally of Jeremy Corbyn, was elected as Labour MP for Leicester East with a majority of 6,019 but had the whip withdrawn last year after being charged.

She denied harassment between 1 September 2018 and 26 April 2020 but after a two-day trial chief district judge Paul Goldspring found her guilty.

“I find she was jealous of the relationship between Lester [Thomas, her partner] and Michelle. Whatever the nature of that relationship, she certainly thought it was something she was not happy with,” he said. “I found Ms Webbe’s evidence to be vague, inconsistent and at times to be illogical. It was shaped around the overwhelming evidence against her she could not innocently answer, but ultimately I found it to be untruthful.”

He said her conduct towards Merritt was “oppressive and was designed to be so”. “In particular threatening to send intimate photographs to her family and to ‘use acid’ are in my view the best evidence of the intent.”

He adjourned sentencing to 4 November and released Webbe, 56, on unconditional bail but warned she could face prison.

A Labour spokesperson said: “The Labour party strongly condemns Claudia Webbe’s actions and she should now resign.”

During the trial Paul Hynes QC, defending, read out a string of character references from figures including Corbyn, the former shadow chancellor John McDonnell and the former shadow home secretary Diane Abbott.

Webbe is a former chair of Operation Trident, which was set up in response to shootings disproportionately affecting black people in London. She was a councillor in Islington between 2010 and 2018 and was a member of Labour’s national executive committee.

She entered the House of Commons in December 2019, winning the seat formerly held by Keith Vaz, the former Labour minister who retired from parliament in the wake of a scandal involving cocaine and sex workers. Last month an official report found Vaz had engaged in “sustained and unpleasant bullying” towards a member of parliamentary staff.

If Webbe receives a custodial sentence of less than a year and her appeal fails, a recall petition would be triggered in her constituency. To prompt a byelection, 10% of the electorate within the constituency would need to sign the petition.

Vaz told the Guardian last month that he would not stand again as a Labour candidate in Leicester East but he is known to still carry some support among the constituency party’s executive and voters.

Westminster magistrates court heard Webbe made a string of unwanted calls to Merritt. Merritt told the court that the calls, most of which were from withheld telephone numbers, made her “worried and unnerved”. “When you’re being called and there’s no one answering it unnerves you, especially as a woman who lives alone,” she said.

After Merritt received a call on Mother’s Day in 2019, in which Webbe identified herself, she decided to contact police, telling them: “She works for the Labour party, she is in the public arena and she’s accusing me of, in her words, ‘slagging around with her boyfriend’, which I’m not. She’s threatened to throw acid over me, she knows where I live, she’s going to send pictures of me and him to my daughters.”

Police officers spoke to Webbe and Merritt and requested that contact should cease, the court heard. But on 25 April last year Merritt received two calls, both eight seconds long and from the same number. Merritt told the court: “I just hung up very quickly. I didn’t want to speak to her.

“[Having received those two calls] I had to somehow prove that she was calling me because until that date all the calls had been no caller ID and the first threat call wasn’t recorded … so I called back the number that was on my phone.”

In the recording, played to the court, Webbe answered the phone and shouted at Merritt: “Why are you still butting in and getting with Lester? Why, why? He’s not your friend, he’s with me and I don’t want you to be in touch with me, I don’t want you to be in touch with him.”

Webbe screamed “get out of my relationship” 11 times, according to the recording. She also said: “I’ve seen all of your posts, I’ve seen all of your naked pictures, I’ve seen all of your relationship with Lester. Otherwise I will tell all of your family and I will show them all of your pictures.”

Giving evidence last month, Webbe, from Islington, denied harassment, claiming she had made “courtesy calls” to warn Merritt not to breach coronavirus rules by meeting Thomas.

Asked whether she had threatened Merritt, Webbe told the court: “I’ve spent my lifetime campaigning for the rights of women, challenging this kind of behaviour. This is not something that’s in my character and it’s not something I would ever do.”

After the verdict, Webbe said she was “deeply shocked”. “I am innocent and will appeal this verdict. As I said in court and repeat now, I have never threatened violence nor have I ever harassed anyone,” she said.

Her lawyer, Raj Chada, added: “We will be appealing this unjust verdict. The recording of the call Ms Webbe made has been taken out of context. We are sure that Ms Webbe will be vindicated at an appeal.”