Boris Johnson has declared that he would campaign to pull Britain out of the EU in a referendum if David Cameron's attempts to negotiate a new deal for Britain in Europe failed.
There would be no point in negotiating for a change of rules in the EU unless the government was serious about being prepared to pull out "and I would be willing to say that," he argued.
His remarks show that on this issue London's Mayor is closer to the Tory right, who are determined to get out of the EU, than David Cameron, who warned earlier in this year against the "pessimists" who refuse to believe that the EU can reform.
The Mayor also expressed bemusement at the rise of UKIP, which he described as the Conservative Party’s 'doppelganger' because, he claimed, they shared identical beliefs. His remarks were very different from how David Cameron described UKIP members eight years ago - as "fruitcakes, loonies and closet racists."
On immigration, the main issue that cost the Tories last week's defeat in the Clacton by-election, won by the Tory defector Douglas Carswell, the Mayor claimed that it should be possible for Britain to negotiate an opt-out of one of the fundamental EU rules that any EU citizen can move freely to any EU state in search of work.
Mr Johnson told the BBC's Andrew Marr programme that border controls are now "the number one thing that we need to sort out" and called for a points system, similar to the USA's, to decide which EU migrants should be allowed into the UK. "I don’t think that’s something our friends in Europe would view with complete horror," he said.
He added: "It’s obviously axiomatic that if we don't get the reform that we need in 2016 or '17, then I think we should campaign to come out... In the past few years I have come to the view that there is a viable and very attractive future for this country with a different arrangement. People are too apprehensive about the possible down side.
"You can't go into a negotiation unless you're prepared to walk away, so it follows that if we don't get the right solution then need a better solution, and I would certainly be willing to say that. There is nothing to fear from this argument and a lot to gain. One of the reasons UKIP has gained in appeal is that people feel they are not being given a proper choice."
The Mayor appealed to former Tory voters who have switched allegiance to UKIP to come back, because there is no real difference between the parties. He said: "There's a very good film starring John Travolta and Nicholas Cage called Face Off, in which basically... the Nicholas Cage guy is fighting a character who looks like himself, the doppelganger. I was listening to Douglas and I wondered what I disagreed with him about, and there wasn't very much.
"So my message to kippers is that it doesn't seem to me there's a lot between us."