A Reform UK MP was jailed for repeatedly kicking his girlfriend 18 years ago, it has emerged.

Former investment banker James McMurdock did not publicly disclose his conviction for assault before being elected and claimed he had “pushed” his partner when details were first revealed this summer.

However, The Times applied to the courts for information from the official record of his sentencing, which it obtained on Wednesday.

It said he was was detained in a young offenders’ institution for 21 days for kicking the victim “around four times”.

The conviction came to light in July when the victim’s mother told Mail Online he had “left marks on her body” and “it took two security guards to pull him off her” during the attack outside a nightclub in Chelmsford, Essex.

At the time, Mr McMurdock apologised for what he described as the “biggest regret of my life” but downplayed the assault as a “teenage indiscretion” and disputed the details of the mother’s account.

In a statement, he said: “A generous person might call it a teenage indiscretion, but I do not expect everyone to be so kind.

“Nearly 20 years ago, at 19 years of age, at the end of a night out together, we argued and I pushed her.

“She fell over and she was hurt. Despite being 38 now and having lived a whole life again I still feel deeply ashamed of that moment and apologetic.”

Mr McMurdock pleaded guilty to assault by beating, according to court records obtained by The Times.

The paper reported that Essex Police refused to provide any documents about the case under freedom of information laws, claiming that to do so would be in breach of data protection rights and could prejudice the detection of crime.

It then obtained the court register through an application to the courts.

The MP’s sentence was not suspended “in light of the serious nature of the offence”, the records say.

A pre-sentence report indicated a “lack of willingness to comply”.

There is no requirement for MPs to disclose previous convictions to the public when standing for election, but Mr McMurdock has been accused of failing to be “properly accountable”.

Labour MP for Nottingham East Nadia Whittome said: “Neither he nor his party (which knew) disclosed the conviction at the election. This man belongs nowhere near law-making and power.

“The refusal to be properly accountable is disturbing… The passive ‘she fell over and she was hurt’.”

Mr McMurdock won the seat of South Basildon and East Thurrock for Reform UK at this year’s election, beating Conservative candidate Stephen Metcalfe by just 98 votes.

He and his party have been contacted for comment.

Essex Police declined to comment.