Emily Thornberry has pledged to put booting Boris Johnson out of No10 ahead of personal ambition as she ramped up her campaign to lead the Labour party.
In a swipe at Jeremy Corbyn the Shadow Foreign Secretary said she would quit if polls showed she couldn’t win or if she lost the confidence of Labour MPs.
Ms Thornberry- the first candidate to official declare - threw down the gauntlet to her rivals to make the same bold promise.
She said that of all the qualities needed in the next leader “what matters most is that we need someone who can win”.
Writing exclusively for the Mirror, she vowed: “If I am elected leader, and if there is any stage where I know – and the polls and my colleagues tell me – that I can’t persuade people, and I can’t get us back into power, then I vow to do what I’ve always done throughout my life.
"I will be loyal to the party I love. So I will stand down and give one of my brilliant colleagues the chance to win instead.”

She vowed to learn from Andrew Little, New Zealand’s Labour leader, who stood down in 2017 when he was flailing in the polls, enabling his successor, Jacinda Ardern, to sweep to power two months later.
The Labour Party is considered less ruthless than the Conservative Party when it comes to sticking with unpopular leaders.
Ms Thornberry’s commitment chimes with comments expressed by Labour MPs - two of whom separately told the Mirror the party needed a “Michael Howard figure” to help it recover from its electoral defeat but not necessarily stay in post for the next election.
The senior politician criticised previous Labour leaders for lacking a “clear vision” and instead argued she has a clear idea where she wants to take Britain but also has “the ability to unite our party and the whole country behind that vision”.
Ms Thornberry, who has faced off Boris Johnson when he was Foreign Secretary and who has stood in for the Leader of the Opposition at Prime Minister’s Questions, insisted she had the “confidence and forensic skill at the Despatch Box” to “blow past Boris Johnson’s bluster and expose him for the feckless, lying charlatan that he is”.
But some in the party worry that a second north London MP leading the party after it was seen by many as out of touch with its traditional voters.
The London MP offended many in 2014 when she tweeted a picture of a house draped in several St George’s flags which was interpreted as snobbery during a by-election in Rochester and Strood.
And she has taken legal action against defeated Labour MP Caroline Flint who accused the shadow foreign secretary of calling leave voters “stupid”. Ms Thornberry accused the former MP of “making s*** up” about her.

Ms Thornberry said she acknowledged she had “flaws” and has “made mistakes in the past and paid a price for them” in reference to the flag incident after which she resigned from the shadow cabinet forcing the hand of a dithering Ed Miliband.
Setting out her stall to succeed Mr Corbyn after the party was decimated in its heartlands, Ms Thornberry promised a shadow cabinet of all the talents so that backbenchers such as Lisa Nandy, who is considering a stab at the top job, would be in poll position to take over the leadership if she failed to win over voters ahead of the next election.
It follows on from remarks she made in the wake of the party’s brutal defeat of the need for “a bridge from where we are now to being elected”.
Jeremy Corbyn, who led the party to its worst election defeat since 1935, has so far resisted calls to quit and instead promised to stay in place until April.
Rebecca Long Bailey is widely considered as the current leadership’s favourite to succeed him while shadow brexit secretary is expected to throw his hat in the ring alongside backbencher Lisa Nandy.
Home Affairs Select Committee chair Yvette Cooper will decide whether to stand over Christmas while backbencher Clive Lewis has already declared.