The Mail on Sunday has "unreservedly" apologised to Ed Miliband after an uninvited reporter went to a private memorial service he was attending for his late uncle.
The paper's editor Geordie Greig promised a full investigation after the Labour leader complained that the reporter attempted to get reaction from his relatives about an article in its sister paper, the Daily Mail, denouncing his late father, Marxist intellectual Ralph Miliband, as the man "who hated Britain".
He said it had been a "terrible lapse of judgment" and two journalists on the paper have been suspended.
"I unreservedly apologise for a reporter intruding into a private memorial service for a relative of Ed Miliband," he said.
"The reporter was sent without my knowledge; it was a decision which was wrong. Two journalists have been suspended and a full investigation is now being carried out.
"I would further like to apologise to members of the family and friends attending the service for this deplorable intrusion. I have already spoken personally to Ed Miliband and expressed my regret that such a terrible lapse of judgment should have taken place.
"It is completely contrary to the values and editorial standards of The Mail on Sunday. I understand that Lord Rothermere is personally writing to Ed Miliband."
Following the incident Miliband has called on the proprietors to mount an urgent inquiry into the papers' "culture and practices".
In a letter to Lord Rothermere, the chairman of Daily Mail and General Trust, he said the decision to send a reporter to the memorial service held yesterday at Guy's Hospital for Professor Harry Keen crossed "a line of common decency".
"My wider family, who are not in public life, feel understandably appalled and shocked that this can have happened," he wrote.
"Sending a reporter to my late uncle's memorial crosses a line of common decency. I believe it a symptom of the culture and practices of both the Daily Mail and the Mail on Sunday.
"There are many decent people working at those newspapers and I know that many of them will be disgusted by this latest episode. But they will also recognise that what has happened to my family has happened to many others.
"Instead, I am writing to you as the owners of the Daily Mail and the Mail on Sunday because I believe it is long overdue that you reflect on the culture of your newspapers.
"You should conduct your own swift investigation into who was responsible at a senior level for this latest episode and also who is responsible for the culture and practices of these newspapers which jar so badly with the values of your readers.
"There are bigger issues for the people of Britain in the midst of the worst cost of living crisis for a century than intrusion into the life of my family. But the reaction of many people to the Daily Mail's attacks on my father this week demonstrates that the way your newspapers have behaved does not reflect the real character of our country.
"It is now your responsibility to respond."
Labour sources said that at the end of the service, Prof Keen's daughter was approached by a woman who shook her hand and offered her condolences, before introducing herself as a reporter from the Mail on Sunday.
The reporter asked whether the daughter wished to comment on the Daily Mail article about Mr Miliband senior and was told "no comment". When the reporter asked again, she was given the same answer, at which point she left.
Earlier Nick Clegg launched an outspoken attack on the Daily Mail, accusing the paper of "overflowing with bile" about modern Britain.
The Deputy Prime Minister said it was "quite understandable" that Mr Miliband denounced the paper after it ran an article about his late father under the headline "The man who hated Britain".
Appearing on his weekly radio phone-in on LBC 97.3, he said that it was the Mail which "excelled" at doing down its own country.
"When I heard the Daily Mail accusing someone of saying that they didn't like Britain… I'm not a regular reader of this newspaper but every time I do open it, it just seems to be overflowing with bile about modern Britain," he said.
"They don't like working mothers, they don't like the BBC, they don't like members of the royal family, they don't like teachers, they don't like the English football team – the list goes on. Talk about kettles and pots.
"I think it was quite understandable that Ed Miliband should react like that because clearly what they had to say about his dad was just out of order. The Daily Mail is free to print what it likes, people like me are perfectly free to say that it's wrong.
"It seems to me that if anyone excels in denigrating and often vilifying a lot about modern Britain, it's the Daily Mail."
Clegg is the latest senior figure from across the political spectrum to voice concern at the way the Mail portrayed the Labour leader's father, who was a Jewish refugee who fled to Britain to escape the Nazis and served in the Royal Navy in the Second World War.