The boss of the Unite union has called on Scottish Labour leader Jim Murphy to resign for "making certain" the Conservatives won the general election.
Scottish Labour lost 40 seats to the SNP last week, leaving just one MP representing the party in Westminster.
Unite's Len McCluskey said the "anti-Scottish card" played by the Tories also led to Labour losses in England.
The BBC understands Mr Murphy faces a vote of no confidence at the Scottish Labour Party Executive on Saturday.
In an interview for Newsnight, Mr McCluskey, general secretary of one of Labour's largest financial backers, said Mr Murphy should "leave the scene".
He said: "There's no doubt in my mind that the late swing in the opinion polls was as a result of the anti-Scottish card that was played by Lynton Crosby and the Conservatives.
"They must have thought that all their Christmases had come early. And we have to examine that.
"In my opinion that has been a major factor in why Miliband is now not the prime minster and I lay the blame for that very squarely at the feet of Scottish Labour.
"Not only have they lost Scotland but I think they've been responsible for making certain that the Conservatives were back in power in Westminster."
Mr McCluskey added: "In Scotland my view is very, very strongly that we have to say to the Scottish people that we're sorry for letting you down, for making you feel abandoned, and Scottish Labour is under new management.
"I think Jim and his colleagues should just leave the scene."
The union leader said that it would be up to Scottish Labour members to choose a new leader.
Mr Murphy has said he is confident of retaining the leadership, despite mounting calls for him to stand aside.
Last week's poll saw the SNP win 56 Scottish seats, leaving Labour with just one - down 40 on the 2010 result.
Former Aberdeen South MP Dame Anne Begg is thought to be among those who have given Mr Murphy their support, saying Mr Murphy was not to blame for 10 years of Labour decline.
A Labour spokesman said: "There is overwhelming support for Jim Murphy across the Scottish Labour Party. We need to work together to rebuild our movement and regain the trust of the people of Scotland."
Pressure has been growing on the Scottish Labour leader to step down.
On Tuesday, the Labour MSP for Cowdenbeath, Alex Rowley, said the party was heading for "disappearance" in Scotland unless Mr Murphy resigned.
The train drivers' union, Aslef, has also backed calls for him to go and Unison Scotland urged a "radical change in approach" by the Labour Party in Scotland and said it "would not oppose" a move to change the leader.
However, Mr Murphy has received backing from Usdaw, which represents shop workers, and Community union.
Community general secretary Roy Rickhuss said: "Now is not the time for self-indulgent political point scoring and internal wrangling. Against a backdrop of rising unemployment in Scotland, we need a united movement and party.
"It is clear that Scottish Labour needs the time and space for Jim Murphy and Kezia Dugdale to lead the rebuilding of our party in Scotland."
Mr Murphy had been an MP for the East Renfrewshire area since 1997 and elected as Scottish Labour in December last year. He lost his seat to the SNP's Kirsten Oswald.