The 40-year-old Swiss-based financier immediately said he would appeal, after Mr Justice Tugendhat ruled that Associated Newspapers had established that the meaning of the May 2010 story was substantially true, despite admitted inaccuracies.
The newspaper's case was that Mr Rothschild took Lord Mandelson, then European Commissioner for Trade, on a January 2005 trip to Siberia in order to impress Mr Deripaska, when he knew, or ought to have known, that if anyone found out about it, Lord Mandelson would have been compromised.
It was also said there were grounds for believing that Lord Mandelson discussed aluminium tariffs with Mr Deripaska and Mr Rothschild encouraged the inappropriate relationship.
The judge, who heard the action at London's High Court without a jury, said he accepted the newspaper's submission that Mr Rothschild's conduct was inappropriate in a number of respects.
"In my judgment, that conduct foreseeably brought Lord Mandelson's public office and personal integrity into disrepute and exposed him to accusations of conflict of interest, and it gave rise to the reasonable grounds to suspect that Lord Mandelson had engaged in improper discussions with Mr Deripaska about aluminium."
He said that, if he had found against the newspaper, he would have made only a modest damages award of £3,000.
Mr Rothschild's evidence was that the presence of Lord Mandelson on the overnight trip to Abakan with himself, another friend, Sebastian Taylor, Peter Munk, chairman of Barrick Gold, and Mr Deripaska, who had a
chalet nearby, was "purely social".
It included visiting a smelter facility and its museum, a banya -
a Russian-style sauna - skiing, five-a-side football, ice-hockey, Russian billiards and entertainment by a Cossack band.
But, the newspaper's counsel, Andrew Caldecott QC, said it was "way beyond the norm" for a Russian oligarch to be flying the trade commissioner, who he did not know well at the time, in his private jet to his private chalet to be wined and dined.
It was accepted that there were recreational moments, but the heart of the purpose of the trip was plainly to allow Mr Deripaska to get to know Lord Mandelson better, and the newspaper said that was inappropriate.
"Businessmen discuss business on ski lifts - and when they are having banyas. You may have a recreational background to a business discussion," he said.
The judge said there were at the very least reasonable grounds to
believe that Mr Deripaska's interest in providing such luxurious and generous hospitality was as the QC suggested.
"And I cannot accept that Mr Rothschild was unable to foresee this at the time he invited Lord Mandelson on the trip. In my judgment Mr Rothschild did appreciate this at the time."
He said that, by "facilitating" the development of a relationship
between the two men, Mr Rothschild was conferring a benefit on, and seeking to please, both of them.
"So far as Lord Mandelson was concerned, the benefit was the trip and the hospitality itself.
"So far as Mr Deripaska was concerned, it was a relationship with the EU Trade Commissioner."
The judge said he accepted that, to a limited extent, Mr Deripaska had already got to know Lord Mandelson at the World Economic Forum that January.
"But it is one thing to meet a person on neutral ground, where neither is giving a benefit to the other, and another thing to welcome a
person into your private plane and your home."
He said it was probable that on the visit to the aluminium smelter, at which they were photographed standing with a guide, the party talked about aluminium, although it did not follow - and he did not find - that they talked about tariffs.
Nor did he find that there were reasonable grounds to suspect that Lord Mandelson talked about aluminium tariffs.
Lord Mandelson and Mr Deripaska were not parties to the action and did not take any part in the trial.
It was emphasised that the case was not about them and the newspaper made no allegations they had done anything wrong.
Mr Rothschild said later that he was disappointed with the ruling although he did not regret bringing the action.
"I brought this action seeking an apology for the Daily Mail's utterly false claim that I had arranged for Lord Mandelson to attend a dinner in Moscow to close a deal between Alcoa and Rusal and that this had caused the loss of 300 British jobs.
"The truth is, as the Daily Mail has now accepted, that I had nothing whatsoever to do with this deal and that it had in any event been completed before Lord Mandelson and I even arrived in Moscow.
"Lord Mandelson's trip to Russia was entirely recreational - as the court has accepted - and Lord Mandelson had obtained clearance for the trip from his office before undertaking it."
A Daily Mail spokesman said: "We are pleased with Mr Justice Tugendhat's judgment, which vindicates our reporting of this story."
He added: "This case is a reminder, at a time when newspapers are
under attack for invading privacy, that the rich and powerful regularly
use the law to prevent legitimate scrutiny of their activities. Had the
Mail lost this case, it could have incurred costs of more than £1 million.
"Not many news organisations, however committed they are to free speech, can afford to risk a loss of that magnitude.
"As Lord Justice Leveson's Inquiry considers the balance between privacy and freedom of expression, the chilling effect on free speech that court cases like this one exert needs to be borne in mind."
PA
