Brexit: Britons Who Support Leaving EU Ahead in Polls Amid Warnings From Economic Leaders

— -- Britain's prime minister has accused those in support of leaving the European Union of making claims that are "complete untruths."

During a rare news conference in London on Tuesday, just hours before registration to vote in the so-called "Brexit" referendum ends, Prime Minister David Cameron, who is campaigning for the U.K. to remain in the E.U., urged voters to “listen to experts,” including the World Trade Organization and the chair of the U.S. Federal Reserve.

Cameron has appeared increasingly uneasy ahead of the June 23 referendum, as the latest official poll by YouGov/GMB gave a slight majority to the pro-Brexit camp -- 45 to 41, with 11 undecided and a small fraction of people saying they would not vote.

The prime minister has been trying to debunk beliefs by the pro-Brexit camp that the U.K. is liable for future Eurozone bailouts, that Britain cannot stop E.U. spending from going up and that leaving the E.U. would save Britain $11.6 billion, among other claims.

Janet Yellen, chair of the U.S. Federal Reserve, has said that a "U.K. vote to exit the European Union could have significant economic repercussions," adding it "could shift investor sentiment."

The director general of the World Trade Organization told The Times, “It could be a few years, it could be decades," for new trade deals with the U.K. to be negotiated, adding that to expect "smooth sailing and quick results would be a high-risk bet."

Cameron urged voters to get involved.

“Don't stand on the sidelines. This matters for you,” he said. “There are no second chances, no re-runs."

The prime minister will join the leader of the U.K. Independence Party, Nigel Farage, in a live referendum special on ITV News, though they will not debate one another. The former mayor of London, Boris Johnson, who is campaigning for the U.K. to leave the E.U. has called on the prime minister to debate him.

"The safer choice is voting to leave, so we can take back control of our money, borders, security, trade and taxes," Johnson said in a joint statement with other pro-Brexit officials. "We think that the public deserve the chance to hear these issues debated face-to-face between the prime minister and a spokesman for Vote Leave so they can judge for themselves which is the safer choice on 23 June."

In terms of latest polls, a majority of the population in other European countries polled believe it is likely that more countries will choose to leave the E.U. in the event of a Brexit.

Results could depend on whether people aged 18 to 24 stay at home in similar numbers as for the general elections, when only four in 10 voted, according to analysis by the Press Association.