The decision of Johnson, a two-term mayor who has been touted as a possible future prime minister, deals a blow to Cameron’s hopes of a united front ahead of what is expected to be a hard-fought referendum.

A new battle for Britain erupted Sunday, with London Mayor Boris Johnson saying he would join the campaign to encourage Britain to leave the European Union. The move posed a direct challenge to Prime Minister David Cameron, who has launched a major push to keep his country within the 28-nation bloc.

The popular, raffish Johnson immediately becomes the most prominent Conservative Party politician to break ranks with fellow Conservative Cameron’s vision of the best course for Britain in a June 23 referendum on EU membership.

The referendum has divided Cameron’s Conservative Party – while most in his Cabinet back his wish to stay, several Cabinet members oppose his stance and are campaigning for the country to break free of EU bureaucracy – a so-called “Brexit.”

The decision of Johnson, a two-term mayor who has been touted as a possible future prime minister, deals a blow to Cameron’s hopes of a united front ahead of what is expected to be a hard-fought referendum.

“The last thing I wanted was to go against David Cameron or the government, but after a great deal of heartache I don’t think there is anything else I can do,” Johnson told reporters outside his London home. “I will be advocating vote “leave” … because I want a better deal for the people of this country to save them money and to take back control.”

So far, no country has ever left the EU, and several European countries outside the bloc are still working to reform their economies and governance systems so they can join.

Cameron made a firm commitment three years ago to give voters a simple “in or out” referendum if he was re-elected in 2015. He was acting to quell divisions within his own party, which has long had a strong vein of anti-Europe sentiment.

Cameron has seen Justice Secretary Michael Gove and several junior ministers defect to the “leave” campaign, but none with the political heft of Johnson.

Two other highly touted possible future Conservative leaders – Treasury Chief George Osborne and Home Secretary Theresa May – have cast their lot with Cameron and will back the “stay” campaign.

Johnson’s decision was not a complete surprise because the 51-year-old former journalist has for several decades criticized the growing EU bureaucracy, first in his newspaper and magazine columns and later from Parliament and city hall.

Striking likely campaign themes, Johnson criticized EU courts for taking too active a role in British affairs and said the entire European project was in danger of spinning out of “proper democratic control.”

The mayor was immediately praised by one of the leaders of the “leave” campaign who have been searching for a charismatic figure with mainstream appeal.

“We’d like to offer the Mayor of London a warm welcome to the Brexit campaign,” said Leave.EU co-chairman Richard Tice. “We share his vision of a UK with full, democratic control of its affairs, and a relationship with Europe based on free trade and voluntary cooperation.”

It is not clear to what degree Johnson will lead the “leave” campaign. He told reporters he did not want to take part in rallies and debates on the issue but would make his feelings clear.

Cameron earlier in the day used a TV appearance to try to convince Johnson to join the “stay” campaign. Cameron cited emerging challenges from Russia and the rise of extremist groups in the Middle East as threats better dealt with as part of an alliance.

“In a world where you have got Putin to the east and ISIS-Daesh to the south, how do you stay strong?” Cameron asked. “By sticking with your neighboring countries, your partners and your friends.”

Cameron argues that those who support leaving the EU because they think it will slow the flow of migrants into Britain are mistaken. He said any trade deal Britain would negotiate with the EU if it left the bloc would have to allow for the free movement of labor in order to satisfy EU demands.

The prime minister also cautioned that it if Britain pulls out, it would take years to negotiate such a deal, since it also needs to make sure that British companies still have access to European markets.

Cameron plans to go to Parliament on Monday to formally set in motion the June 23 referendum.

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• Courant Times - Saturday 6 February 2016