The train drivers’ union is backing Jeremy Corbyn to become leader of the Labour Party.
Aslef said the Islington North MP has the "character, vision and policies" needed to win back power for the party.
General secretary Mick Whelan said: "Jeremy understands what Labour has to do to win back the hearts, the minds and the votes of ordinary working people in Britain."
Mr Corbyn said: "Aslef is an excellent campaigning union and I am delighted to receive their endorsement.
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"I have campaigned alongside them for the public ownership of the railways, for better rights for people at work, and against the policies of austerity that are damaging our country."