Len McCluskey, head of the Unite Union, says it is because of Scottish Labour that the Conservatives won the General Election.
Speaking to Newsnight, he said party leader Jim Murphy had “made certain” that Labour would lose, and that he should now “leave the scene”.
Support for the party plunged across Scotland last week, when it lost 40 of its 41 seats to the SNP. Murphy is reportedly facing a vote of no confidence on Saturday, following a large number of calls for him to resign in light of the result.
"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,” McCluskey said. "I think Jim and his colleagues should just leave the scene."
Not everyone believes Murphy is so much to blame, however – former Aberdeen South MP Dame Anne Begg is among those standing by Murphy, saying he cannot to blame for 10 years of Labour decline. Murphy himself has expressed confidence that he can hold on to leadership.
McCluskey also believes the Conservative party's “anti-Scotland card” contributed to Labour's poor performance last week.
"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,” he said. "They must have thought that all their Christmases had come early.”
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.