In a statement delivered outside May's Downing Street residence, she said, "It is and will always remain a matter of deep regret to me that I have not been able to deliver Brexit."
She will step down as UK's premiere on June 7 when the formal contest to see who will replace her will begin. During this time, May will serve as the caretaker prime minister.
In an emotional address, May said, "I will resign as leader of the Conservative and Unionist Party on Friday, 7th June."
She added, "The process for electing a new leadership should begin in the following week."