The row over the Daily Mail's article on Ralph Miliband is threatening to overshadow the Privy Council meeting on press regulation this week.
Ed Miliband has refused to link his battle with the Mail to the two competing Royal Charters going before the council, but analysts expect the row to make a compromise agreement between the newspaper industry and parliament much harder to achieve.
In an interview with the Independent, Miliband again tried to untangle the two issues, saying one was personal while the other was political.
"I did this because I wanted to stand up for my dad," he said.
"I spoke out as a son who did not want to see his late father being smeared. He fought for our country, owed his life to our country, and I was not prepared to let the lie that he hated our country stand."
Around 200 protestors gathered outside the Daily Mail's offices in Kensington yesterday to demonstrate against what they termed "grotesque acts of journalism".
Meanwhile a You Gov poll for the Sunday Times found 72% of the public believe the Mail was wrong to say Ralph Miliband "hated Britain", while 69% of people in general and 57% of Daily Mail readers wanted the newspaper to apologise.
Labour said the Mail's account of the article was unravelling after a senior figure at the newspaper admitted the article attacking the Labour leader's father should have been labelled as comment.
"Sometimes articles which are comment should be labelled and made clear that they are comment," City editor Alex Brummer said.
"So perhaps it should have said 'comment' on it to make clear absolutely it was comment instead of a special report.
"But I think people understood that it wasn't just reporting; there was an element of commentary to it. Just maybe the labelling wasn't quite what it should have been."
A Labour spokesperson responded: "Six days on from their original article, the Daily Mail's defence is crumbling.
"For the first time, they have been forced to admit significant problems in the way they dealt with the article 'The man who hated Britain'.
"They should now finally have the good grace and decency to acknowledge their grotesque error of judgment and apologise."
The Privy Council will make its decision on Wednesday, with Lord Justice Leveson breaking his post-inquiry silence the following day with an evidence session to the Commons media committee.