The Communication Workers Union (CWU) has called on Scottish Labour leader Jim Murphy to resign following the party’s near wipeout in Scotland.
Mr Murphy is expected to face a vote of no confidence at a meeting of Scottish Labour’s executive in Glasgow on Saturday.
CWU is affiliated to the Labour Party and represents workers in post, telecoms, mobile, and financial services companies including BT, Capita, EE, O2, Parcelforce, the Post Office, Royal Mail, Santander and UK Mail.
A spokesman said: “CWU is calling for the resignation of Jim Murphy as Scottish Labour leader following the party’s performance in the general election, which was particularly disastrous in Scotland.
“It is clear that the Labour Party needs to rebuild in Scotland and CWU believes that will be done most effectively under new leadership.”
Labour members are split on Mr Murphy’s future, with senior MPs and MSPs publicly defending the recently-ousted East Renfrewshire MP, insisting Labour’s problems pre-date his appointment as leader and were too deeply entrenched to turn around in a few months.
But trade union members, many of whom backed left-wing candidate Neil Findlay in the leadership contest last year, have urged Mr Murphy to go.
Members of the Unison trade union who are affiliated to Labour has said that it is “unprecedented for a party leader not to stand down after such a defeat”.
While they stopped short of calling for him to resign, they said if there was a wider call for a new leader in the party, they would not oppose it.
Leaders of the trade union Usdaw offered their support to Mr Murphy, saying a second leadership contest in a year in the party in Scotland would be “navel-gazing”.
Left-wing Scottish Labour activists have urged Jim Murphy to resign to allow the party to reconnect with the public without the distraction of a rejected MP at the helm.
A meeting of about 100 activists organised by the Campaign For Socialism reached the almost unanimous decision that “Jim Murphy must go” on Wednesday.
Mr Murphy was only elected Scottish Labour leader in December last year after his predecessor Johann Lamont quit, accusing the party in London of treating Scotland like a “branch office”.
He has already said he wants to stay in the post as the “long-standing problems that led me to stand for leadership of this remarkable party in the first place” still need to be tackled.
But two members of his shadow Scottish cabinet have quit their posts in the wake of the election result, with Alex Rowley, who had been the party’s local government spokesman at Holyrood, arguing a “‘fundamental change in direction and strategy” is needed.
The Cowdenbeath MSP said it would be a mistake for Mr Murphy to remain in the post and lead Labour into next year’s Scottish elections.
Neil Findlay, who had stood against Mr Murphy for the post of Scottish leader, quit his post as fair work, skills and training spokesman on Saturday.