The hereditary peer said yesterday that he was prompted to make the move by his admiration for Tony Blair, adding: "They [Labour] arecarrying out so many policies I believe in."
He announced his decision to stunned silence from colleagues at a party held on Wednesday night to mark the 30th anniversary at the Mail of its veteran columnist Lynda Lee Potter.
Lord Rothermere's cousin, Vivian Harmsworth, said yesterday that the hereditary peer had been an "illegitimate Conservative" and had never before taken his seat, despite being listed as a Conservative Peer in the Parliamentary directory, Dodd's.
So far he has not taken the Labour Party whip and his cousin could not confirm if he would be voting with the Labour Party on every issue.
If Lord Rothermere did vote with Labour he could find himself voting for his own abolition. He is a hereditary peer and Labour has made a manifesto commitment to remove hereditary peers' voting rights.
"His views have accumulated over the last few years" said Vivian Harmsworth, "and he has taken a positive view on so many things that the Government has done so quickly."
Lord Rothermere's move is likely to have serious implications for the editor of the Daily Mail, Paul Dacre, who backed the Conservatives in the election. Acknowledging that his decision would raise speculation about Mr Dacre's future, Lord Rothermere said in a statement: "Paul Dacre is a great editor and I have very good relationships with him but we do not always share exactly the same views on politics."
"Basically Rothermere thinks Blair is the man," said a Mail source. "There is definitely a feeling upstairs that Dacre screwed up and should have backed Blair."
The public announcement of Lord Rothermere's defection was made in a diary item in yesterday's London Evening Standard. A source said the story "dropped mysteriously into the paper from upstairs". The Evening Standard supported Labour at the election.
Tony Blair began wooing the Mail papers two years ago when he had a one- to-one lunch with Lord Rothermere. In the end the Daily Mail put the Union Jack on its front page the day before the election and told its readers to vote Conservative for the sake of British independence from Europe.Reuse content