Britain leaving EU would be preferable if David Cameron cannot secure reforms, claims Boris Johnson

The Mayor of London believes the 28-nation bloc is 'suffering from a crisis of economic underperformance'

London Mayor Boris Johnson believes a 'generous exit' could be negotiated (Getty Images)

Boris Johnson is to insist Britain’s economy could flourish outside the European Union, arguing the country has nothing to fear from tearing up its membership.

The Mayor of London’s provocative intervention will pile pressure on David Cameron as he embarks on talks aimed at wresting powers back from Brussels.

But Mr Johnson will say that leaving the 28-nation bloc would be “attractive” if Mr Cameron is not successful and there would be “no reason for hostility or rancour on either side”. He will argue: “If we got it right, we could negotiate a generous exit.”

Britain would be able to gain access to the single market, resulting in a “combination of a lower regulatory burden and undiminished trade access would cause exports to boom”, he will say in a speech in London.

He will back a report from his economic adviser, the banker Gerard Lyons, who concludes that quitting would be preferable to remaining in an unreformed EU.

Mr Johnson will claim the EU is “suffering from a crisis of economic underperformance, and a related collapse of political trust” and accuse it of throwing half a generation of young Europeans on the scrapheap.

The Prime Minister has always predicted victory in his negotiations and said he would argue for a Yes vote in the referendum he has promised on EU membership if the Tories win next year’s election.