Prime Minister David Cameron accused those campaigning for Britain to leave the European Union of lying, saying on Tuesday he had been struck while watching the news on TV by their incorrect claims.
Britons vote in a referendum on June 23 on whether to remain in the 28-member bloc, a momentous decision for the economy, trade, politics, defense and migration in Britain and beyond.
A recent spate of polls has suggested that the "Leave" campaign may be gaining momentum.
Cameron, who is leading the "Remain" campaign, convened an unexpected news conference at short notice during which he tried to move the focus away from immigration, his opponents' most potent line of attack which dominated the debate last week.
He stressed the strong backing his side has received from a wide range of bodies like the International Monetary Fund, the Bank of England and the World Trade Organisation, and accused Leave campaigners of telling multiple "untruths."
"It is irresponsible. It is wrong. It is time that the Leave campaign was called out on the nonsense that they are peddling," said Cameron, appearing on a rooftop terrace with sweeping views of the London skyline and the Houses of Parliament.
"I was watching the news last night and it just came over to me so clearly that there is such a contrast between the weight of independent expert opinion ... and a series of assertions from the Leave campaign that just simply aren't right."
Cameron cited recent warnings on the economic impact of an EU exit made by Federal Reserve chair Janet Yellen, Hitachi's chief executive and the head of the WTO.
His last-minute decision to hold the news conference signaled a break from what has so far been a carefully choreographed campaign.
That shift prompted claims from some parts of the media and the Leave campaign that Cameron has been rattled by the narrowing polls.
"The In campaign is in a blind panic," said Vote Leave spokesman Douglas Carswell, a lawmaker from the anti-EU UK Independence Party.
Asked by a reporter whether he was worried he was losing, Cameron said: "Not at all. What I'm worried about, what I'm concerned about, is that people are being told things that aren't correct."
Cameron had previously held back from criticizing the main figures in the official Leave campaign, many of whom are senior members of his Conservative Party, including former London mayor Boris Johnson and Justice Secretary Michael Gove.
But on Tuesday, he cited several arguments made by Johnson, Gove and others and dismissed them.
"It's so important to say to people: 'don't take a decision to leave the EU and damage our economy on the basis of clearly false information that you're being given,' " he said.
Additional reporting by Kate Holton and David Milliken, writing by William James and Estelle Shirbon; editing by Stephen Addison.