In a two-page editorial, the popular tabloid said that “by any calculation” Britain would be bound to face higher tariffs, turmoil in the financial markets and a period of uncertainty if there was a vote to Leave on June 23.
It was a move that surprised many in Westminster and in the media in light of the paper’s largely pro-Brexit coverage of the referendum, particularly by its sister paper the Daily Mail.
A Loughborough University study of media coverage of the referendum found The Daily Mail was one of the most partisan papers favouring a Brexit of any on Fleet Street.
Journalists speculated the endorsement reflected internal politics within Associated Newspapers between The Mail On Sunday, edited by Geordie Greig, and The Daily Mail, edited for more than 20 years by Paul Dacre.
In its editorial, the Mail on Sunday accused Brexit campaigners who argued that the UK could thrive in “splendid isolation” outside the EU of peddling a “dangerous illusion”.
“The human head knows that, especially in the world we now inhabit, our deepest desires must somehow be moderated to suit the increasingly tough reality of a competitive world in which, though still a great nation, we no longer have the power or the wealth which once allowed us to live in splendid isolation,” it said.
“Those who would have you believe in the plucky Little England of the past are selling a dangerous illusion.”
It said the “single-minded leaders” of the It Leave campaign were so intent on securing independence from the EU they were “ready to pay any price for it” – even it meant other people losing their jobs.
“So eager are they for a divorce that they are prepared to sacrifice a large chunk of our income, and trade down on living conditions, in order to walk out into a rose-tinted future of ‘freedom’. Though it is interesting that they have been careful to make no such a declaration.
“Perhaps their reluctance to do so is they know that others - who do not share their passion for estrangement - may also have to pay the price, perhaps with their jobs.
“It is not much of a vote-winner to say to the electorate: ‘You may have to suffer for my ideals.’”
The Daily Mail has yet to declare for either side in the referendum. The Sunday Times has come out for Brexit, putting it at odds with its daily counterpart The Times, which has backed Remain.
People noted that the differing positions of papers in the same ownership reflected how the decision fell to editors, not proprietors.
It follows people on Twitter claiming that Rupert Murdoch was behind The Sun's decision to back a Brexit.
The Sunday Telegraph, meanwhile, has come out in favour of Brexit.
In an editorial it said: “On balance, however, we believe the Leave campaign has articulated an ambitious vision for Britain as an independent nation, once again free to make its own decisions. Remain, by contrast, has resorted to grim pessimism.
“Without the EU, they imply, the UK would be diminished and a diplomatic pariah, scrabbling to put together trade agreements while our economy flounders.”
The newspaper accused Chancellor George Osborne of making “unconscionable threats” over pensions and “dire predictions” on trade agreements that, with other issues, amount to an “unremitting tide of gloom”.
It said the UK does not need a trade deal to buy and sell goods, and branded the idea of spending the next decade in economic limbo as new relationships are hammered out as “risible”.
Declaring that the EU “belongs to the past”, it said: “Leaving the EU does not mean leaving Europe. A vote for Brexit on Thursday will not change our geography. Just as our island story has been intertwined with that of the Continent, often with bloody and tragic consequences, it will continue to be so.”
It added: “There is a world beyond Europe that the Remain camp simply ignores. A world that offers enormous opportunities for Britain to be a global player once more.
“The case for leaving is not negative and jingoistic. It is optimistic and hopeful. It is the case for a strong, independent and outward-looking Britain.”