With several recent opinion polls suggesting momentum is with the "Leave" camp, Cameron is making a string of television appearances to try to convince people to back "Remain" on June 23.
A string of global institutions including the International Monetary Fund (IMF) and G7 have backed Cameron's argument that Britain's economy would be damaged by Brexit.
But "Leave" supporters argue that Britain could thrive outside the EU, where they say it would be freed of red tape.
The prime minister told the BBC on Sunday that there could be a "lost decade for Britain" after a vote to leave the EU as the political system gets gummed up with negotiations.
"I think we'd be looking at a decade of uncertainty," he added. "It would suck the energy out of our government and our country."
A Sunday Times/YouGov online poll found that Cameron's "Remain" campaign is lagging the "Leave" side by 42 per cent to 43 per cent.
Averaging out the last six opinion polls, both sides are tied on 50 percent, according to academics at the What UK Thinks project. Their figures exclude undecided voters.
"Nobody knows what these polls are saying," Cameron told the BBC, while stressing he was optimistic of victory.
But Farage insisted that they showed a real movement towards his side.
"There has been a shift in the last fortnight," the UKIP leader said.
"People have had enough of being threatened by the prime minister and the chancellor and I think collectively people are beginning to put two fingers up to the political class.