UKIP's Nigel Farage told Italy's Corriere della Sera in an interview published on Saturday that British Prime Minister David Cameron should resign even if the 'Remain' camp won by a narrow margin in a June 23 referendum on EU membership.

Bookmakers still predict Britons will vote to stay in the EU, while opinion polls have see-sawed - though one on Friday gave a 10 percentage point lead to the 'Leave' side, for which Farage is a prominent campaigner.

"We can't lose this referendum ... there will be a Big Bang of British politics: nothing will remain the same," Farage said. He said he would "destroy the old EU", together with 5-Star leader Beppe Grillo.

"On June 19 the 5-Star movement elects the mayor of the capital and changes Italy. On June 23 Britain leaves the EU and changes Europe. We will trigger a domino effect. After us, other northern European countries will leave, starting with Denmark," he said. "The EU is about to collapse, disintegrating in several pieces."