Prime Minister David Cameron accused those campaigning for Britain to leave the European Union of lying, saying on Tuesday he had to challenge their views after watching TV news bulletins.

The prime minister called an unexpected news conference at short notice during which he tried to move the focus away from immigration in an increasingly heated campaign before the June 23 referendum on whether to remain in the 28-member bloc.

It was a tactic that Cameron, who is leading the Remain campaign, also used in a television debate with anti-EU campaigner Nigel Farage, whom he accused of wanting to turn the country into "Little England" rather than Great Britain.

Opinion polls have suggested that the Leave campaign may be gaining momentum.

"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.

He dismissed suggestions from the Leave campaign and some local media that he was worried he was losing. He said he was concerned that "people are being told things that aren't correct".

"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, keen to get the focus of the campaign on the economy, 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 signalled a break from what has so far been a carefully choreographed campaign.

"The In campaign is in a blind panic," said Vote Leave spokesman Douglas Carswell, a lawmaker from the anti-EU UK Independence Party.

Television debate

Later on Tuesday, in front of an audience on ITV television, Cameron pressed his criticism of those campaigning to leave the European Union.

He accused Farage, leader of the UK Independence Party and a leader of one of the groups wanting a British exit, or Brexit, of wanting to damage Britain's economy.

"I hope that when people go to vote on June 23, they think about...the sort of country that we want to build together and they vote to say that we don't want Little England of Nigel Farage. We want to be Great Britain," he said.

Cameron had previously held back from criticising 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.