David Cameron has been dealt a fresh blow as a former Tory leader said the EU renegotiation has "met with failure".
Michael Howard, once a political mentor to the Prime Minister, says he believes Britain should vote to leave to "shake Europe's leaders out of their complacency".
Lord Howard said Britain "would be sorely missed" if it quit the EU and and suggested that "there would be a significant chance that they would ask us to think again" if voters backed Brexit.
In an article for The Daily Telegraph, he wrote: "I had hoped that when the Prime Minister announced his intention to commence negotiations for a new relationship between the UK and the EU he might be able to achieve fundamental reform along these lines.
"When he spoke, at the outset of the negotiations, of the need for fundamental reform, I believe he may have had something of this kind in mind.
"It is not his fault that those efforts met with failure. It is the fault of those EU leaders so mesmerised by their outdated ambition to create a country called Europe that they cannot contemplate any loosening of the ties which bind member states.
"There is only one thing that just might shake Europe's leaders out of their complacency: the shock of a vote by the British people to leave."
It comes amid claims that the world's most powerful economies are poised to warn against Britain quitting the European Union, following talks with George Osborne.
Finance ministers are meeting in Shanghai and the Chancellor is expected to press for the G20 to signal its concerns about a possible Brexit.
David Cameron, meanwhile, will head to Wales as he continues to tour the UK putting the case for staying in 28-member bloc.
He will tell voters that nearly half of Welsh trade is with the EU and the nation will be "better off" if the UK remains in.
Britain's future in the EU is not on the formal agenda for the G20 meeting but Mr Osborne is expected to have talks with counterparts on the sidelines.
Officials at the talks told the Financial Times they expected there would be a reference to Brexit in the official communique.
"I predict it will (be included) because the UK will want it to," one said.
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