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Boris Johnson will urge David Cameron to fight harder to defend the City of London this week in a keynote speech on Britain’s relationship with the European Union.

The Mayor will publish a report on Wednesday by his economics adviser Gerard Lyons that will warn of the danger of the eurozone countries ganging up against London.

In an interview with the Evening Standard today, Mr Lyons warned of an “unsympathetic attitude” in Europe towards London’s financial services sector and the peril of being “frozen out”.

“If we just take the last 12 months for instance, the financial transactions tax and the EU bankers bonus approach, they basically are indicative of what you could say is an unsympathetic attitude towards the City and towards the financial sector,” said banker Mr Lyons.

“I think there is a real need to speak up a bit more for the financial sector as well as London more generally in terms of this debate and the EU reform process.”

He urged ministers to champion the City as fiercely as the French defend their agriculture industry. He added: “The City cannot be complacent about what could unwind if we stay in the EU.”

The Mayor’s highly-charged intervention on European policy is being widely seen as a milestone towards a return to Westminster politics at next year’s general election.

Mr Johnson today confirmed that he believes Britain could be better off quitting the EU unless it reforms.

“When you look at the cost of EU social policy, the stagnation of the EU economies, the continuing absurdities of some Brussels regulation, we are plainly getting to the stage where it might well be better to quit an unreformed EU than to stay in,” he wrote in the Telegraph.

The Lyons Report will say London’s best hope for prosperity is a reformed EU, which would see the capital’s earnings grow by 2034 from £350 billion to £640 billion. However, if EU reform ran into the sand, London’s GDP would only grow to £495 billion.

Quitting the EU with a good trade deal and pursuing global trade policies would grow the capital’s GDP to £614 billion, making London better off outside an unreformed EU. Leaving the EU on poor terms and with inward-looking trade would be the worst option, limiting GDP to £430 billion.

Mr Lyons told the Evening Standard that the higher priority for EU reform should be ensuring that the euro-zone does not act against the interests of non-euro countries like Britain.

“It is vital for the City that, if we remain in the EU, the non-euro members like the UK are not squeezed out by a shift of regulation from the UK to the EU and that this regulation becomes even more unsympathetic towards non-EU members, including the City and the UK.”

Mr Cameron has promised to hold and in/out referendum by 2017, after renegotiating Britain’s terms of membership. Mr Cameron has said he intends to win successful reforms and campaign for an “in” vote, but the Mayor’s intervention will increase pressure on him to make a more explicit threat to recommend an “out vote” if negotiations fail.