In a letter addressed to the Prime Minister, Putney MP Justine Greening said she was "proud" to have been the first education secretary to have gone to a comprehensive school, and the first openly gay female Cabinet minister.
But the arch-Remainer, who has called for a second referendum on the issue of Brexit, has announced she is quitting as a Conservative MP, saying she will not stand for the party at the next election.
She maintained that Prime Minister Boris Johnson has offered the country a "lose-lose" situation by threatening a general election.
Alongside her letter of resignation (which you can read in full below), she tweeted that she would "continue to speak up for our community in Parliament & campaign on #socialmobility".
In her resignation letter to Mr Johnson on Tuesday she said: "I'm proud to have been the first comprehensive school educated Secretary of State for Education and the first openly LGBT female Cabinet Minister."
Ms Greening entered Parliament in 2005 as MP for Putney, taking back the traditionally Conservative seat.
In her role as education secretary she said she wanted to make sure every child had a chance in life, striving for social mobility.
Read here he full letter to Boris Johnson:
Dear Prime Minister,
For over 14 years I have had the privilege of serving by constituents and community in Putney, Roehampton and Southfields. It has also been my honour to serve in Government, as a Treasury Minister and running three successive departments over six and a half years in Cabinet - as Secretary f State for Transport, International Development and Education, alongside being a Minister for Women and Equalities. I’m proud to have been the first comprehensive school educated Secretary of State for Education and the first openly LGBT female Cabinet Minister.
In all my roles, fighting for equality of opportunity and social mobility has been at the heart of my efforts. Social mobility is something that has characterised my own personal life growing up in Rotherham as well as my political life rooted in Putney. It has always mattered to me more than any particular role or job title. As you know, last year I left Cabinet to focus my effort on this crucial issue for the country.
I believe it is now time for me to move on to other opportunities and challenges outside of Parliament. I wanted to let you know that I will not be standing as a Conservative candidate.
Instead I will focus my efforts on the Social Mobility Pledge which I set up las year to help bring more opportunities to more young people. Through this, I am already leading a coalition of over 350 companies employing almost 3 million people. As Parliament remains gridlocked on Brexit, I believe I can achieve more positive change on the ground by working outside Parliament on this very practical initiative. Through the Social Mobility Pledge, I will continue to play a role in public life in Britain, making a difference for the better.
Like many people locally, I am deeply concerned by the Brexit approach taken by your Government. To be clear, I remain the MP for my community and I am determined to work hard and represent my constituents on all the issues they care about, including on Brexit. However, it’s clear to me that whoever is next elected to represent our constituency, which voted over 70% Remain, they will similarly need to put our community views on Brexit first, ahead of any conflicting party interest.
Above all I would like to massively thank residents for giving me the privilege of serving as out Member of Parliament, elected over four General Election victories. I’m also grateful to all those who have actively supported me in our local Conservative party and beyond. I never planned to become an MP. I did it because I believe in Britain, our democracy and giving people a voice and a choice. This time has been hugely special and the greatest honour of my life.