UK Labour MP Angela Eagle said she was withdrawing from the race to oust party leader Jeremy Corbyn, saying she would give her support to rival Owen Smith instead in order to boost the chances of a change of leadership.
Eagle triggered the contest last week by challenging Corbyn, but said she had since received fewer nominations from Labour politicians than Smith.
"It is in the best interests of the Labour Party that we now come together so we can have one candidate," she told reporters on Tuesday.
Corbyn has been battling to keep his job since Britain's shock vote on June 23 to leave the European Union, holding on despite a vote of no confidence backed by three-quarters of the party's members of parliament.
Efforts to keep him off the leadership ballot paper by forcing him to secure a minimum 51 nominations from Labour MPs and members of the European Parliament - a 20 percent support prerequisite for his challengers - failed last week.
Corbyn's rivals are concerned that he cannot win a national election and will not give Labour a strong enough voice in Britain's forthcoming Brexit negotiations.
But in refusing to resign, Corbyn has cited strong support among grassroots members. He will stand against Smith in a vote of Labour activists and party supporters, with a result due on September 24.
"We have a Labour Party at the moment that is not working. We have got a leader that doesn't have the confidence of his members of parliament and isn't reaching out to the country," Eagle said.
"We need to have a strong and united Labour Party so that 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.
"We are going to be in lock-step together arguing for an effective, united Labour Party doing its job."