The London mayor announced that he will campaign for a leave vote after concluding that Cameron's deal will not deliver the reformed EU he promised.

His decision is a big blow to the prime minister who had pleaded with him earlier yesterday to avoid 'linking arms' with opposition Nigel Farage and George Galloway who are for Britain to exit EU.

Speaking outside his home in north London, the mayor said his decision had been 'agonisingly difficult'.

But he added: "I would like to see a new relationship based more on trade, on cooperation, with much less of this supranational element. So that is where I’m coming from and that is why I have decided, after a huge amount of heartache, because the last thing I wanted was to go against David Cameron or the government, I don’t think there is anything else I can do.

Downing Street issued a low-key response, with one of its spokesmen saying "Our message to everyone is we want Britain to have the best of both worlds: all the advantages of the jobs and investment that come with being in the EU, without the downsides of being in the euro and open borders."

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