British Prime Minister David Cameron has told supporters of the "In" campaign that European Union membership makes Britain economically stronger and safer.

"It's about guaranteeing our economic security here in the United Kingdom. It's the most important thing there is. I'm absolutely convinced that our economic security will be better if we stay in a reformed European Union and it will be seriously at risk if we were to leave," Cameron said.

However, leading "Brexit" campaigner, former Mayor of London Boris Johnson, said that all attempts to reform the 28-member bloc were "hopeless". He said that those backing "Brexit" had the "passion and courage and a sense of duty" that their opponents lacked.

An opinion poll found that only 21 percent of respondents in the survey agreed that Cameron was more likely to tell the truth about the EU than Johnson, while 45 percent said Johnson was more believable than Cameron. Britons will go to the polls on June 23 for an in-out referendum on the country's EU membership.