Last week, Haigh became the first minister to resign since the Labour government took office in July.
On Thursday evening, Sky News and The Times revealed details of her past conviction.
Haigh subsequently issued a statement giving further details of the incident, which took place in 2013 when she was working for insurance company Aviva.
She said she had reported a mugging to the police and that a work mobile phone was one of the items that had been stolen.
She later said she found the handset in a drawer at home.
Turning on the phone “triggered police attention”, she said, and the matter was taken to magistrates’ court for making a false report to police.
Haigh said: “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.”
She received a conditional discharge six months before becoming an MP in the 2015 general election.
Early on Friday, the morning after the details of the conviction emerged, Haigh sent a resignation letter to the prime minister, saying she did not want to become a distraction.
The BBC has been told by Whitehall sources that she had told Sir Keir about her conviction when he picked her to be in his shadow cabinet in 2020.
She did not tell the government’s propriety and ethics team about it when she became a member of the cabinet after Labour won July’s general election.
She believed it was sufficient to have disclosed her spent conviction to Sir Keir when Labour was in opposition, the BBC has been told.
A Downing Street spokesperson refused to say what Sir Keir knew about the conviction, only that he accepted her resignation after “further information” emerged.
