LONDON (AFP) - A British newspaper suspended two journalists and apologised to opposition Labour leader Ed Miliband on Thursday, after it sent an uninvited reporter to a family memorial service in a "terrible lapse of judgment".

The Mail on Sunday's apology to Ed Miliband comes days after its sister newspaper the Daily Mail sparked uproar with an article in which it branded Miliband's dead father Ralph, a Marxist theorist, "The Man Who Hated Britain".

The Mail on Sunday apologised "unreservedly" to Miliband after it emerged on Thursday that one of its reporters had sneaked into a private memorial for his uncle Harry Keen in a bid to get them to comment on the Ralph Miliband controversy.

"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," the Mail on Sunday's editor Geordie Greig said.

"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."

The apology came after Miliband had written to Lord Jonathan Harmsworth, the proprietor of the two right-leaning tabloids, to make a formal complaint.

"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," he wrote in the letter to Harmsworth.

Miliband's mention of "culture and practices" harks back to the terms of a major inquiry into the British press launched in the wake of the phone-hacking scandal at Rupert Murdoch's News of the World tabloid in 2011.

He said he was not going to bother complaining to Britain's "widely discredited" Press Complaints Commission, which is set to be replaced by a new regulatory body in the wake of the scandal.

But the creation of the new watchdog has been delayed by a disagreement between the British press and lawmakers over the extent of state involvement.

Miliband has already complained to the Daily Mail over its article on Saturday about his father, which came after the Labour leader unveiled an increasingly left-leaning stance at his party conference last month.

Critics of the newspaper pointed out that Ralph Miliband, who died in 1994, served in the British navy during World War II after fleeing Belgium as a Jewish refugee in 1940.

Ed Miliband defeated his own brother David - foreign minister under prime minister Gordon Brown - in a battle for the Labour leadership in 2010.