Angela Eagle has withdrawn from the contest for Labour's leadership in order to back Owen Smith as a "unity candidate" to challenge Jeremy Corbyn.
The Wallasey MP was the first to mount a public challenge to the Labour leader, but decided to withdraw after it became apparent that Mr Smith had secured more nominations from party MPs and MEPs.
Former shadow work and pensions secretary Mr Smith will now go forward to fight Mr Corbyn in a ballot of party members and supporters which ends on September 24.
Speaking at Westminster, Ms Eagle said she would be backing Mr Smith in the contest, adding: "We are in lock-step together, arguing for a united Labour Party."
She continued: "We have a Labour Party at the moment that is not working, we've got a leader that doesn't have the confidence of his Members of Parliament and isn't reaching out to the country.
"We need to have a strong and united Labour Party so we can be a good opposition, take the fight to the Conservative Government and heal our country.
"So I am announcing tonight that I will be supporting Owen in that endeavour with all my enthusiasm and might."
Owen Smith has received 88 nominations from Labour MPs and two from MEPs in the party's leadership contest, therefore clearing the hurdle of 51 nominations to become a candidate.