"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.

"It is completely contrary to the values and editorial standards of . I understand that Lord Rothermere is personally writing to Ed Miliband."

Miliband, locked in a row with sister paper the over a report saying his late father hated Britain, wrote in his letter that he spoke at the memorial service, held at Guy's hospital in London for his uncle, Professor Harry Keen, a distinguished doctor.

"It was an event in a room on the 29th floor of Guy's hospital which was attended only by family members, close friends and colleagues," he wrote. "I was told by one of my relatives late yesterday evening that a reporter from had found her way into the event uninvited.

"I also discovered that, once there, she approached members of my family seeking comments on the controversy over the 's description of my late father as someone who 'hated Britain'."

He added: "Sending a reporter to my late uncle's memorial crosses a line of common decency."

Labour sources insisted the main service was a private event and it is understood the reporter learned about the service from somebody's Facebook page.

The reporter at one point held the hand of one of the grieving relatives and then twice asked her to comment on Ralph Miliband, the source said. But she refused to do so.

The source said that Greig confirmed the reporter had been sent by the newspaper's executives and on Wednesday evening was deeply apologetic.