BORIS Johnson has claimed the price of Britain quitting the European Union is "lower than it's ever been" despite bleak warnings about the potential impact of a vote to sever ties with Brussels.

The mayor of London insisted that "in an ideal world" he would want to stay in a reformed EU, but he played down the risks of a so-called Brexit.

The senior Tory repeatedly refused to rule out spearheading the campaign for the UK to leave the EU but insisted he had "great faith" in David Cameron's ability to negotiate better terms of membership.

Mr Johnson's intervention, during a visit to Osaka in Japan, came as a campaign to keep the UK in the EU used a high-profile launch event to deliver a stark warning about the potential damage to the UK from a decision to leave.