Nadine Dorries is to stand down as Culture Secretary after Liz Truss replaced Boris Johnson as Conservative leader.
An ultra-loyalist to Mr Johnson, Ms Dorries backed Ms Truss during the leadership election but she said she believed it was the right time to go.
Ms Truss will fly to Balmoral to meet the Queen for the formal handover of power from Mr Johnson and become Britain's 56th Prime Minister later today.
It is understood Ms Dorries was given the opportunity to carry on in Cabinet but had chosen instead to return to the backbenches.
She is expected to be given a peerage in Mr Johnson’s resignation honours list, triggering a by-election in her Mid Bedfordshire constituency.
In her resignation letter to the outgoing PM, she said: "I am humbled that your successor has extended her confidence in me by asking me to remain as Secretary of State for DCMS."
But she said "after much reflection" she had decided to step down, adding: "I have personally assured our soon-to-be prime minister that I will be better-placed to support her from outside of the Cabinet."
During the leadership campaign, Ms Dorries was an outspoken critic of Rishi Sunak – in one controversial tweet likening him to Brutus stabbing Julius Caesar over the way he had turned on Mr Johnson.
As a minister she was involved in drawing up legislation to curb social media companies through the Online Safety Bill and led controversial moves to privatise Channel 4.
This comes as Priti Patel resigned as Home Secretary and said she will not serve in Liz Truss ' Cabinet.
She said it was "my choice to continue my public service to the country and to the Witham constituency from the backbenches" as she jumped before she was pushed.
She was widely expected to get the sack when Ms Truss reshuffles her Cabinet - with hard-line Attorney General Suella Braverman tipped to be the next Home Secretary.
But Ms Patel tried to suggest that she was choosing to step down rather than being forced out by the new Prime Minister.
In her letter to Mr Johnson, which was shared on social media, she wrote: "It has been the honour of my life to serve as Home Secretary for the last three years.
"I congratulate Liz Truss on being elected our new leader, and will give her my support as our new Prime Minister.
"It is my choice to continue my public service to the country and the Witham constituency from the backbenches, once Liz formally assumes office and a new Home Secretary is appointed."
Ms Patel has been Home Secretary since Boris Johnson became Tory leader in July 2019 and was International Development Secretary under Theresa May before she was sacked for holding unauthorised meetings with Israeli officials in 2017.
She has been a divisive figure at the Home Office, presiding over the controversial Rwanda deportation scheme as she struggled to tackle the rising numbers of asylum seekers arriving in Britain in small boats.
