But many Conservatives said the treatment of Ralph Miliband was no different to that given to Margaret Thatcher when she died earlier this year.
The Mail refused to apologise to the Labour leader for an article about his Marxist late father, entitled 'the man who hated Britain'.
Former Tory MP Louise Mensch weighed in with a picture of Miliband posing with an anti-Thatcher campaigner:
When Ed Miliband publishes his rebuttal piece in the Daily Mail, here's the perfect photo to accompany it. pic.twitter.com/Iaxi5hx7pv— Louise Mensch (@LouiseMensch) September 30, 2013
And Karl McCartney, MP for Lincoln, retweeted this one:
Plenty of Tory supporters agreed:
It came as an emotional Miliband said he had been "appalled" by the Mail's article and the followup it published alongside his response.
Cameron said he had not read the article, but added: "All I know is that if anyone had a go at my father, I would want to respond very vigorously."
And Johnson added: "You're entitled to stick up for your family."
Speaking at a fringe debate organised by The Huffington Post UK, Tory MP Zac Goldsmith said it was "odd for a newspaper to judge a man on the basis of the history of his family when that newspaper is owned by a family that did more to pursue the Nazi cause prewar than any other [publication]".