Events in Falkirk "have betrayed the values of our party" Ed Miliband wrote today in The Observer.
"The practices we have seen should be unacceptable in any political party," he added. "But they are certainly unacceptable in the Labour party."
Harriet Harman confirmed today that Labour will impose a spending cap on selection campaigns and Miliband is expected to lay out reform plans in a speech this week.
Calling to "mend, not end" the party's relationship with trade unions, he said moves needed to be made to ensure future selections "are always fair, open and transparent" and give greater prominence to individual union members.
The Labour party will place a spending cap on selection campaigns, Harriet Harman announced today.
Speaking on BBC1's Andrew Marr Show, Harman said the move was part of a "significant change" in the party's relations with trade unions, following Labour's row with Unite over allegations of ballot rigging.
The Shadow Culture, Media and Sport Secretary said she had taken out a second mortgage on her home to fund her campaign to be deputy leader, because she was not backed by a union.
– Harriet Harman
It is very important that people cannot be ruled out of a contest because they can't get the backing of a union or have their own independent funds.
Labour leader Ed Miliband has got to restore trust or pay the price at the election, former Labour minister David Blunkett wrote in the Daily Mail.
Mr Blunkett said that Mr Miliband needed to "reclaim the moral high ground" after a row over candidate selection.
– David Blunkett
Whatever the nature of the private discussions which took place between Ed and Tom, the decision that he should step down from his role in the selection of candidates was not only inevitable but essential.
The last few days have demonstrated that Ed Miliband is prepared to grasp this opportunity and ensure that Labour will go into the Election with the trust of the British people restored.
If he doesn’t, we will pay the price at the ballot box.
Stop 'playing into the hands' of the Tories, Unite general secretary Len McCluskey said today, amid an highly-damaging row over candidate selection.
In an article for the Sunday Mirror, Mr McCluskey urged Mr Miliband to "step back from the brink of a ruinous division".
He wrote: "Labour's attack on Unite is alienating tens of thousands of decent people from the party at a time when we should all be pulling together to take on this rotten government and its policies causing so much misery.
"Don't led Blairite diehards like Lord Mandelson be your back-seat drivers...and don't let the Tories set our agenda.
"They are wrecking Britain. Their only hope is to wreck Labour too. This week the party's leaders have played into their hands. It is time to reunite".
Scotland's First Minister has urged the country's Labour leader to "break her silence" in the row over the selection of an election candidate in Falkirk.
Alex Salmond said: "Labour's meltdown over its selection procedure in Falkirk has sent Ed Miliband's leadership into a tailspin.
"But it also exposes the empty rhetoric at the heart of claims that Scottish Labour is in charge of its own affairs, and the silence of the party leadership north of the border.
"Johann Lamont has stood by silent and immobilised while Labour have imploded in Falkirk. It has exposed the total pretence that she leads Labour in Scotland. It is London Labour who have been in control as Falkirk Labour spin out of control".
Ed Miliband said he wanted to "mend...not end" Labour's relationship with the trade unions as he promised further party reforms to prevent a repeat of a highly-damaging row over candidate selection.
The Labour leader, who is engaged in a bitter dispute with the leader of the party's biggest union donor over claims it sought to rig a ballot, said he wanted to increase the involvement of individual members.
But he dismissed suggestions he was looking at ending the historic formal ties with the unions.
The behaviour of Unite officials in Falkirk was "amateur and irresponsible", former Labour MP for Falkirk Eric Joyce wrote in the Guardian, after reports that trade union Unite paid for votes to secure friendly candidates at the General Election.
Mr Joyce wrote: "Over the years, trade unions have used their putative power sensibly. They've understood that party rules create the possibility of serious dysfunction if they choose to overexert their potential muscle.
"In Falkirk I've found them to be a stabilising influence in partnership with the Labour party. Until now.
"The amateur, hubristic and irresponsible actions of a small number of Unite officials at the top of the organisation will require some rules to be changed to prevent another Falkirk".
Speaking to ITV News, Ed Miliband said he would defend the right of individual trade union members to try and become Labour members of parliament. But the Labour leader said he would not defend any instances of corrupt or bad practice.
Unite general secretary Len McCluskey said Labour are caught up in "anti-union Tory hysteria" but he still backs Ed Miliband and the party, during an interview with Sky News.
He added that he had no confidence in Labour's review into the Falkirk selection controversy.