Meanwhile, Nigel Farage continues to back the Brexit
The world's biggest economies have warned the UK against leaving the European Union, saying the economic "shock" would be felt around the world, Chancellor George Osborne has said.
The Chancellor said the top finance officials from the Group of 20 nations had unanimously backed the campaign to keep the UK in the EU at a meeting in Shanghai.
"Here at the G20, finance leaders and central bank governors of the world's biggest economies have raised serious concerns about the risks posed by a UK exit from the EU," Mr Osborne said.
"They have concluded unanimously today that what they call the shock of a potential UK vote to leave is among the biggest economic dangers this year.
"If that's their assessment of the impact on the world economy, imagine what it would do to the UK."
Their agreement was included in a communique released after the meeting.
It comes as the Prime Minister continued his tour of the UK, setting out his case for staying in the 28-nation bloc.
He told farmers in County Antrim that Northern Ireland is better off in a reformed EU - saying 60% of its exports go to the union: "I'd say it comes down to a very simple argument - which is do we want a greater United Kingdom inside the European Union with the safety, strength and prosperity I have spoken about? Or do we want a great leap in the dark?"
Mr Cameron has vowed to visit all regions in the UK as he campaigns ahead of the EU referendum on 23 June.
He is up against key Conservative figures who have joined the Leave campaign, including Justice Secretary Michael Gove and Mayor of London Boris Johnson.
Both Mr Cameron and Mr Osborne both took thinly veiled swipes at Mr Johnson once again on Saturday, who has been accused of flip-flopping on the issue.
Mr Johnson implied in his Telegraph column last week that leaving the EU could trigger a better offer from Brussels and a second referendum.
"EU history shows that they only really listen to a population when it says 'no'", he wrote.
He later told The Times that he was not in favour of using a no vote as leverage to secure further reforms - saying "out means out".
Mr Cameron said politicians showing uncertainty over the issue should set of "alarm bells" for voters: "Many of those who want to ask you to vote to leave actually are not really sure whether they want to leave or not. I think that should set alarm bells going in the minds of voters about the potential dangers and risks that we could face if we leave," Mr Cameron said.
Meanwhile, UKIP leader Nigel Farage has placed concerns about immigration at the heart of his pitch to voters ahead of the EU referendum.
In a speech at his party's spring conference in Llandudno, north Wales, Mr Farage said 23 June could be "independence day" if the country voted to leave the EU, giving back the UK control of its laws and borders.
The UKIP leader said the true scale of immigration would be a "shock" if official statistics properly revealed it, and challenged Home Secretary Theresa May to a TV debate ahead of the vote.
Mr Farage warned the migration crisis would only get worse if the country kept ties with Brussels - and claimed it could leave the UK vulnerable to a Paris-style terror attack, or a repeat of the incidences of sexual harassment seen in the German city of Cologne on New Year's Eve.
Brandishing his "European Union" British passport, Mr Farage criticised the EU's free movement rules: "I do not believe that we are being told the truth about the number of people coming to this country. I believe that the true figures actually would shock us. Mass migration into Britain on this scale is not good for our country. It is not good for our quality of life, it is not good for social cohesion in our society, and our population inexorably headed towards 70 million or 75 million will not make this a better, richer or happier place to be. But as EU members there is nothing we can do about it."
But despite Mr Farage's rallying call, the row between the rival Leave camps cast a shadow over the gathering.
Mr Farage's support for Grassroots Out and Leave.EU is at odds with his sole MP Douglas Carswell and former deputy chairman Suzanne Evans, who back Vote Leave.
At a Vote Leave fringe event Ms Evans, who was sacked from her role days before the conference, highlighted research that suggests Mr Farage was one of the "least trusted voices" on Europe.
Mr Carswell was also there, but party sources dismissed as "tosh" reports he could be thrown out of the party to boost Grassroots Out's bid for the Electoral Commission's official designation in the referendum campaign.
Mr Farage said in response to Ms Evans' comments: "She is an ordinary member UKIP, she can say what she likes. I couldn't care less."