It's Boris v Dave for the EU referendum as Johnson joins the exit campaign

The London Mayor’s decision gives the ‘leave’ side the help of one of the few politicians with widespread public recognition and cross-party appeal and is a major shot in the arm after months characterised by squabbling between different elements of the ‘out’ camp.

But his decision also only served to further highlight the extent of the split in the Conservative Party on the issue, a position mirrored in Yorkshire where Tory MPs and MEPs have already lined up on opposite sides of the debate.

Mr Johnson defied a last minute plea from the Prime Minister to support Britain’s continued EU membership on the back of the reform package secured in Brussels last week.

The London Mayor said David Cameron “did fantastically well” with his reform negotiations in the timescale available.

He added: “But I don’t think anybody could realistically claim that this is fundamental reform of the EU or Britain’s relationship with the EU and it’s my view after 30 years of writing about this I have the chance to do something.”

Mr Johnson’s decision prompted suggestions that he was motivated by a belief it will help garner support within the Conservative Party for his future leadership ambitions rather than out of principle.

Liberal Democrat leader Tim Farron said: “Boris has had more positions on Europe than the Karma Sutra.

“It reflects years of bitter infighting within the Tory party that even the Mayor of London won’t campaign to stay in Europe. He should get a backbone and support British business and the city by campaigning to stay in Europe.”

Mr Johnson’s declaration of support for a ‘leave’ vote was the culmination of a weekend which saw Conservative division on the referendum laid bare.

In Yorkshire, Jason McCartney and Andrew Percy joined fellow Conservative MPs Andrea Jenkyns, Philip Davies and David Davies in calling for voters to back a British exit from the European Union while Kevin Hollinrake, Julian Smith and Craig Whittaker were among those declaring their support for ‘remain’.

Yorkshire’s two Conservative MEPs are also split on the issue with Timothy Kirkhope urging his party colleagues to back the Prime Minister.

He said the negotiations of Britain’s membership terms had “changed the mindset of both the institutions in Europe and the Governments of the other Member States” .

Mr Kirkhope, the Conservatives’ Justice Affairs spokesman in the European Parliament, said EU co-operation was also crucial on security matters.

“Leaving the EU would put us all in great peril, in my view and would be of great benefit only to these people who aim to do us harm,” he said.

In contrast, Amjad Bashir, elected in 2014 as a Ukip MEP before defecting to the Conservatives, said David Cameron had been “let down” by other European leaders on his attempts to secure reforms.

“As things stand, we have more to gain from leaving than from staying and whichever way Britain now votes in the referendum, we will be in a better place than we were before the renegotiation started,” he said.