Tim Roache, the General Secretary of the GMB Trade Union unexpectedly resigned tonight.
In a statement, the union said Mr Roache had been "suffering with ill health for some time now".
But some members of staff were unaware of his abrupt resignation until late tonight, shortly before it was made public.
The statement read: "Tim has unfortunately been suffering with ill health for some time now and has made the difficult decision to stand down from his role, that he does not feel able to continue, in leading our union going forward.”
The Mirror understands senior figures inside the union urged Mr Roache to go, although the circumstances surrounding the decision are unclear.
He was seen as increasingly critical of the party’s left wing, leading to friction both inside and the union and in the wider party.
Mr Roache, 58, had worked for GMB since 1979 and was elected General Secretary in 2015.
