Announcing the decision – after an all-day hustings with all the leadership campaign teams – Len McCluskey, Unite’s general secretary, praised Ms Long-Bailey’s “brains and brilliance”.
And he rejected suggestions that she bore some of the responsibility for Labour’s general election disaster, having helped write its rejected manifesto – blaming the defeat on Brexit.
In a clean sweep for left-wing candidates, Unite, as expected, threw its weight behind shadow justice secretary Richard Burgon in the deputy’s race.
Ms Long-Bailey, the shadow business secretary, said she was “honoured”, praising the union’s “future candidates programme” and saying: “It was Unite, my trade union, that supported me to achieve my potential.”
She already has the backing of the Bakers, Foods & Allied Workers Union, so simply needs support from one of Labour’s many affiliates to formally enter the contest.
Sir Keir, the shadow Brexit secretary, is the favourite to win the leadership race when the result is declared on 4 April.
Mr McCluskey said: “Becky has the brains and the brilliance to beat Boris Johnson. She is standing for unity, socialism and the determination to make Johnson’s term in office short-lived.
“Unite is also confident that Richard will make a superb deputy to Becky, displaying the qualities that have long been absent from that post – pride in our values, a passion for our party to succeed and, above all, loyalty to their leader.”
The reference to “absent qualities” was a clear reference to Tom Watson, the former deputy blamed by Corbyn’s team for undermining his leadership.
Emily Thornberry, the shadow foreign secretary, is struggling to enter the contest’s final stage, requiring the support of 33 local parties in the absence of union support.
Earlier, Labour announced it had cancelled its leadership hustings in Leeds on Saturday after Sir Keir pulled out because his mother-in-law is critically ill in hospital after a serious accident.
Ms Long-Bailey is seeking to shed her image as the outgoing leader’s natural successor, pledging to build a shadow cabinet from all wings of the party and talk up working-class aspiration.
However, she also vowed to bring in open selections – forcing all sitting MPs to fight to keep their seats – a traditional left-wing cause.