Boris Johnson has said that Britain would celebrate 'Independence Day' if the country leaves the European Union, on the first official day of campaigning for Britain's referendum on EU membership. At a Vote Leave rally in Manchester on 15 April, the outspoken Mayor of London said that he saw a positive future for the UK outside of the European Union.

"If we hold our nerve and we are not cowed and we vote for freedom and we vote for democracy on June 23. Then I believe that this country will prosper and thrive as never before," Johnson said.

He attacked the Remain campaign for projecting a negative argument to promote staying in the bloc.

"They say it's crap but we have no alternative. That's what they say, that's their line. But folks I'm afraid that's their line – but we do have an alternative – and it's a glorious alternative. A relationship with Europe based not on the whims of unelected bureaucrats, but on co-operation of elected governments," he said.

Johnson added that staying in Europe was comparable to being stuck in a minicab with a driver who "doesn't have perfect command of English and going in a direction we frankly don't want to go".

In a separate BBC interview, Johnson also branded US President Barack Obama a "hypocrite" for backing David Cameron's campaign to keep the UK in the European Union, saying that the US would never accept compromises on its sovereignty the way Britain had done with the EU.

A YouGov Poll for the Times Newspaper on 14 April showed that 39% of voters backed Britain staying in the 28-member bloc, with the same number wanting to leave. 5% said they would not vote, while 17% were undecided, the online poll of 1,693 people carried out on 11-12 April found.

A poll-of-polls of voters' intentions from the non-partisan group NatCen Social Research, shows that Remain and Leave votes are equally split at 50% each.

Britain decides whether its future lies in the EU or outside of it in a referendum on 23 June.