LONDON (Reuters) – Even if David Cameron clinches a deal from 27 other European Union leaders on Friday, Britain’s prime minister will still have one important person to win over at home – London Mayor Boris Johnson.

The rumpled mayor, who has charmed voters with a buffoonish persona that obscures fierce ambition, has so far refused to endorse a proposed EU deal to keep Britain in the 28-member bloc despite a 40-minute personal briefing from Cameron.

“I’ll be back – no deal as far as I know,” quipped Johnson, wearing a woollen hat, after the meeting in Number 10 Downing Street.

He has said he would “come off the fence with deafening éclat” after Cameron clinches his deal.

Betting odds show gamblers believe Johnson will throw his weight behind EU membership, though if he opposed Cameron on the EU he could deepen a split in the ruling Conservative Party and increase support for the leave campaign.

Guardian political commentator Rafael Behr said Johnson was displaying “a pantomime of procrastination” to get the best price from Cameron before finally offering support to stay in.

“The Machiavellian explanation has him withholding support for the prime minister’s plan when a draft was published this month to raise the price of his eventual endorsement,” Behr wrote. “Hardline sceptics have long suspected that he is a pro-European at heart.”

An Ipsos MORI poll showed Johnson, 51, is second only to Cameron when it comes to swaying public opinion on Europe. One in three voters said Johnson would be important in helping them decide which way to vote, the poll showed.

“Boris Johnson has a broad range of appeal – both to in and out supporters, Conservatives and non-Conservatives, and whether people have already decided or may change their minds,” said Gideon Skinner, Head of Political Research at Ipsos MORI.

If Cameron gets a deal at the Brussels Summit, he will return to London on Friday to present it to a cabinet meeting of senior ministers and recommend the government backs a vote to stay in the EU.

After that, ministers who disagree with Cameron’s position will be able to speak out against EU membership.

Johnson, instantly recognisable thanks to his riotous platinum-blond hair, has asked Cameron to give additional guarantees that the British Parliament is sovereign over EU laws, though it is unclear how Cameron will do that.

Some Conservative Eurosceptics say more than half of Cameron’s lawmakers could vote to leave. But although several ministers are considering backing an EU exit, most of Cameron’s most senior colleagues are expected to back his deal.

Johnson is no stranger to EU headquarters in Brussels, where he was appointed as a correspondent for the right leaning Daily Telegraph in 1989 aged 24. In his reports, he cast then Commission President Jacques Delors as the creator of a European superstate that was plotting to undermine British interests.

But as mayor of London since 2008, Johnson has frequently argued that Britain benefits from being in the EU to influence its decisions. He has been known to wax lyrical on both sides of the debate.

On the one hand, he wrote in the Daily Telegraph this month of the “woeful defects of the EU. It is manifestly undemocratic and in some ways getting worse. It is wasteful, expensive and occasionally corrupt.”

On the other hand, he told Germany’s Spiegel magazine last year: “We can’t leave Europe. We’re part of the European Continent. What is the English Channel? It’s a primeval river that got slightly too big.”

Still, he hedged his bets: “I don’t think it would be the end of the world either if we left. I’m going to see what kind of reforms we can achieve.”

(Reporting by Guy Faulconbridge; editing by Peter Graff)