Suella Braverman was “amazed” and “in denial” over being forced to resign for breaching the ministerial code, according to sources.
Ms Braverman stood down as then PM Liz Truss’s home secretary on 19 October after admitting to a “technical infringement” by sending an official document from a personal email account.
A number of sources have disputed Ms Braverman’s version of events.
Rishi Sunak is facing questions over reappointing her six days later.
The new prime minister defended his decision to make Ms Braverman his home secretary, saying “she made an error of judgment, but she recognised that, she raised the matter and she accepted her mistake”.
The BBC has spoken to several people with knowledge of the events surrounding Ms Braverman’s resignation.
A number of them dispute Ms Braverman’s claim to have reported her mistake to the cabinet secretary – the head of the civil service – as soon as she realised.
When confronted about her transgression she attempted to play down and explain away what had happened, sources suggested.
Ms Braverman had emailed a draft written ministerial statement on immigration policy to her close political ally, Conservative MP Sir John Hayes, using her personal email instead of her official government account.
The document made reference to plans to “make changes to the visa system to support economic growth” and to “ensure our visa system supports priority growth projects” and “extending the high potential individual visa route”, the BBC understands.
Ms Braverman had intended to copy in Sir John Hayes’s wife, who works in his office. But she chose the wrong recipient and instead sent it to a staff member of another Conservative MP, Andrew Percy, early in the morning of 19 October.
When Mr Percy became aware of what had happened, he approached the chief whip, who is responsible for party discipline. The chief whip in turn passed the issue on to Number 10 and the Cabinet Office.
It’s understood Mr Percy’s office also made Ms Braverman aware of the error shortly after the mistaken email was received.
Cabinet Secretary Simon Case advised then prime minister Ms Truss that “it was an open and shut case of the ministerial code being breached”, according to one government insider. He had not by then been approached by the home secretary herself, according to sources.
Sir Keir advises Rishi Sunak: “He should sack her – that would be the strong thing to do. That’s what I would do if I was PM.”
Downing Street has insisted it won’t comment on the “timeline” of events.
One source told me “initially [Ms Braverman] was in a state of denial” when Ms Truss made it clear that afternoon she would have to resign.
“She was saying it was a minor thing,” the same source added.
Another said Ms Braverman was “amazed” that Ms Truss took the view she did.
A source close to the home secretary said any suggestion she was confronted by the cabinet secretary with evidence of the breach was false, adding: “In advance of the meeting with the prime minister, she had communicated it proactively via official channels, to the cabinet secretary.”
Instead of waiting for the resignation to be choreographed by Number 10, Ms Braverman published her resignation letter on Twitter last Wednesday evening.
In the letter, Ms Braverman said she had “serious concerns” about Ms Truss’s government’s commitment to reducing migration numbers.
Labour is calling on Mr Sunak to sack Ms Braverman, claiming that she poses a potential security risk.
The opposition party is also demanding “urgent reassurances” that the leaked email did not contain “market sensitive information”.