Labour leader Ed Miliband said he would not allow the party's name to be "undermined by the behaviour of a few individuals" as the row over the Falkirk selection controversy continues to escalate.
Speaking at a charity event in London today, Mr Miliband said: "The Labour Party I lead will select its candidates in a fair and transparent way. We will act without fear or favour.
"Instead of defending what happened in Falkirk, Len McCluskey should be facing up to his responsibilities. He should not be defending the machine politics involving bad practice and malpractice that went on there, he should be facing up to it."
Mr Miliband said: "It's wrong because we had members being signed up without their knowledge, bad practice, malpractice and frankly instead of defending that kind of thing Len McCluskey should be condemning it."
He added: "Let nobody be in any doubt, there is only going to be one outcome to this: the Labour Party will act in a way that upholds the integrity of our party, the integrity of our party members and the integrity of ordinary trade union members.
"I will not allow the good name of the Labour Party to be undermined by the behaviour of a few individuals."
Labour leader Ed Miliband has turned on the trade union Unite - his biggest financial backer - whose votes helped him become leader.
Tom Watson has announced his resignation from the shadow cabinet, amid a row about the influence of the union Unite on the Labour party.