Transport Secretary Louise Haigh has admitted pleading guilty to a criminal offence relating to a police investigation over a mobile phone she claimed was stolen.

In a statement, Haigh said she told police she had lost her phone during a mugging on a night out in 2013 but later found it had not been taken.

She said it was a “genuine mistake” but had been advised by a lawyer “not to comment” during a police interview. The police then referred the case to the Crown Prosecution Service, she said.

She said she pleaded guilty to making a false report to police at magistrates’ court six months before becoming an MP in the 2015 election, and received a discharge – the “lowest possible outcome”.

A discharge is a type of sentence given to someone who is guilty of an offence but where the court decides not to impose a significant punishment.

Whitehall sources have told the BBC that the transport secretary declared her discharge on appointment to the shadow cabinet when the Labour Party was in opposition.

Details of the specific offence were not immediately clear.

In a statement, Haigh said: “In 2013 I was mugged while on a night out. I was a young woman and the experience was terrifying.

“I reported it to the police and gave them a list of what I believed had been taken – including a work mobile phone that had been issued by my employer.

“Some time later I discovered that the mobile in question had not been taken. In the interim I had been issued with another work phone.

“The original work device being switched on triggered police attention and I was asked to come in for questioning.

“My solicitor advised me not to comment during that interview and I regret following that advice.

“The police referred the matter to the CPS and I appeared before magistrates’ court.

“Under the advice of my solicitor I pleaded guilty – despite the fact this was a genuine mistake from which I did not make any gain. The magistrates accepted all of these arguments and gave me the lowest possible outcome – a discharge – available.”

Haigh has been Sheffield Heeley MP since 2015, and held a number of shadow ministerial and shadow cabinet roles before becoming transport secretary when Labour won the election in July.

She is also a former Special Constable, a volunteer police officer with full powers of arrest, and served with the Metropolitan Police until 2011.

The BBC has asked the Department for Transport to clarify the offence she pleaded guilty to and whether the discharge was conditional, or unconditional.

The chair of the Conservative Party said the prime minister has “serious questions” to answer about the matter.

Nigel Huddleston said in a statement: “These are extremely concerning revelations about the person responsible for managing £30bn of taxpayers’ money.”