A newly elected Reform MP said he was “deeply sorry” after it emerged he had spent a week in a young offenders institution nearly 20 years ago for assaulting a former girlfriend.

James McMurdock, who won the South Basildon and East Thurrock seat at the general election by just 98 votes from Labour, admitted the incident and said it had happened on a drunken night out.

The Daily Mail, which first revealed the details, said it had spoken to the victim’s mother, who said Mr McMurdock had “left marks” on her daughter’s body.

In a statement, Mr McMurdock said: “This is the biggest regret of my life and I wish I could go back and fix things.”

Reform returned five MPs at the general election. The most high-profile was Nigel Farage, who won in Clacton, but little was known about Mr McMurdock, who was described by his leader as a “paper candidate” who had joined the party only in May.

Rival parties said Mr McMurdock, 38, should have explained his past to the electorate before polling day.

Gavin Callaghan, the Labour leader of Basildon Borough Council, posted on X: “Law breakers can’t be law makers. This is a deeply disturbing story about our new MP in South Basildon and East Thurrock.

“If Reform UK knew about his criminal record before the election, why were they not honest about this with the public?”

According to the Daily Mail, Mr McMurdock pleaded guilty to one count of assault at Chelmsford Crown Court. He was working as a barman at the time.

In the report, the woman’s mother said there was “no way he should be an MP”.

Reform told the newspaper it was aware of the conviction but “believe strongly that people can change their lives”.

In response, Mr McMurdock, now a father of four, explained the offence happened when he was 19 and that he had “pushed her”.

His statement continued: “Despite us both being very drunk, I handed myself into the police immediately and admitted my fault. I was charged for what I did, not for what has been claimed, and I faced the consequences then and paid for my action in full.

“It was the realisation of what happened that night and the shame I felt over it that led me to turn my life around.”

Addressing his role as a new MP, his statement added: “As a Member of Parliament, sworn in on the Holy Bible, my job is to represent with empathy and care all our constituents.

“I hope you believe me when I say I know how growth and redemption feels in ways that many others cannot.

“To this day I feel deeply sorry for the hurt I caused that cold, drunken evening 20 years ago,” he added.