Transport Secretary Louise Haigh has become Labour’s first Cabinet casualty after it emerged she pleaded guilty to a criminal offence.

Ms Haigh had incorrectly told police in 2013 that a work mobile phone was stolen.

In a letter to Prime Minister Sir Kier Starmer, she said she was “totally committed to our political project” but that it would be “best served by my supporting you from outside government”.

The Sheffield MP stepped down a day after admitting 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 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.

Downing Street has named justice minister Heidi Alexander, external as her replacement.

In her letter to the Prime Minister Ms Haigh said that her appointment as the “youngest ever” female Cabinet member “remains one of the proudest achievements of my life”.

However, she appreciated “whatever the facts of the matter” the issue would “inevitably be a distraction” .

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.

Ms Haigh caused concern for Labour ahead of international investment summit after describring P&O ­Ferries a “rogue operator” because of its history of fire and rehire practices.