Louise Haigh has resigned as transport secretary after pleading guilty to a criminal offence related to incorrectly telling police that a work mobile phone was stolen in 2013.

In a letter to the prime minister, Haigh said she was “totally committed to our political project” but that it would be “best served by my supporting you from outside government”.

Her resignation comes a day after she admitted that she told police she had lost her phone during a mugging on a night out 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 a magistrates’ court six months before becoming an MP in the 2015 election, and received a discharge – the “lowest possible outcome”.

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

In her letter to Sir Keir Starmer, the prime minister, Haigh said that she appreciated “whatever the facts of the matter” that the issue would “inevitably be a distraction”.

Haigh said that her appointment as the “youngest ever” female Cabinet member “remains one of the proudest achievements of my life”.

A discharge is a type of conviction where a court finds the person guilty but does not give them a sentence because the offence is considered very minor.