Speaking in Downing Street, May said it had been "the honour of my life" to serve as Britain’s second female prime minister. Her voice breaking, she said she would leave "with no ill will, but with enormous and enduring gratitude to have had the opportunity to serve the country I love".
The prime minister listed a series of what she said were her government’s achievements, including tackling the deficit, reducing unemployment and boosting funding for mental health.
But she admitted: "It is and will alway remain a matter of deep regret to me that I have not been able to deliver Brexit."
May’s announcement came after a meeting with Graham Brady, the chair of the backbench 1922 Committee ‐ which was prepared to trigger a second no-confidence vote in her leadership if she refused to resign.
Her fate was sealed after a 10-point "new Brexit deal", announced in a speech on Tuesday, infuriated Tory backbenchers and many of her own cabinet ‐ while falling flat with the Labour MPs it was meant to persuade.
The leader of the House of Commons, Andrea Leadsom, resigned on Wednesday, rather than present the Brexit bill to parliament.
A string of other cabinet ministers had also expressed concerns, including Sajid Javid, Jeremy Hunt, Chris Grayling and David Mundell.