"Jeremy Corbyn is unable to provide the leadership that this party needs - I believe I can. I am not a Blairite, I am not a Brownite and I am not a Corbynista, I am my own woman," she said.
Corbyn was chosen as Labour leader last year on a wave of support from the party's most left-wing activists. But he has clashed with its lawmakers who overwhelmingly passed a no-confidence motion in his leadership. Many have accused him of not campaigning strongly enough to keep Britain in the European Union in last month's referendum.
Also on Monday, U.K. lawmaker Andrea Leadsom announced she was pulling out of the race to become the new leader of Britain's ruling Conservative Party.
The announcement means Interior Minister Theresa May is the sole candidate for the job and will likely succeed David Cameron as British prime minister.