David Cameron and Boris Johnson are now clear rivals in the battle for Britain's future in Europe. But who else is on their side?
Here are a dozen key political figures voting alongside the Prime Minister for Britain to remain and a dozen leading names backing the Mayor of London in a bid for a permanent EU 'Brexit' ahead of the June 23 referendum.
Prime Minister David Cameron is leading the campaign to remain in the 28-member bloc after striking a new membership deal with EU counterparts following marathon talks in Brussels.
Mr Cameron said he managed to win "special status" for the UK, with restrictions on benefits for migrants and an exemption from "ever closer" union.
Chancellor of the Exchequer George Osborne said remaining in the EU under Britain's renegotiated position would provide "the best of both worlds".
He said Britain could get the benefits of free trade and the security of the EU without the costs of the eurozone and said a vote to leave was a "huge leap in the dark".
Home Secretary Theresa May has said the EU was "far from perfect" but that it was "in the national interest" for Britain to remain part of it.
She said: "For reasons of security, protection against crime and terrorism, trade with Europe, and access to markets around the world it is in the national interest to remain a member of the European Union."
Foreign Secretary Philip Hammond was seen as a possible candidate for defection but threw his support behind the Prime Minister after the renegotiations.
He said David Cameron's Brussels reform deal "tilts the balance" in favour of Britain staying in.
Defence Secetary Michael Fallon has dismissed Brexit campaigners' claims that Britain can be fully sovereign and self-governing outside of the EU as an "illusion".
He said "however frustrating" the EU is it was better for Britain to be "at the table, shaping (key) regulations".
Health Secretary Jeremy Hunt offered his support for the remain campaign after the PM's renegotiations.
He said the deal "means sovereignty and (the) pound (are) protected" and said Britain's voice for freedom, democracy and human rights issues was "stronger if we stay".
Development Secretary Justine Greening praised the PM for a "good deal" and signalled her intention to vote in favour of staying in, but added it was "time for millions of British people to have their say in this referendum".
Immigration Minister James Brokenshire confirmed he would be backing the campaign to keep Britain in the EU, which given his department brief will have been strongly welcomed by the PM.
Labour leader Jeremy Corbyn said his party would be campaigning for Britain to remain in the EU because "it brings investment, jobs and protection for British workers and consumers".
But he criticised the PM's Brussels deal, saying it was "largely irrelevant to the problems most British people face" and a "missed opportunity to make the real changes we need" in Europe.
Shadow foreign secretary Hilary Benn - in contrast to his late father Tony Benn - has said Britain is stronger by being in Europe but confirmed that he would not share a platform with the PM to advocate a vote to stay.
He also told ITV's Good Morning Britain that Boris Johnson's reasons for supporting the Brexit campaign read like "a decision in search of a justification".
Labour London mayoral candidate Sadiq Khan has said Londoners' "jobs, livelihood and security" would be put at risk if Britain votes for a Brexit.
The Tooting MP is divided with Zac Goldsmith on the issue and accused his leading rival of "placing dogma ahead of Londoners' interests".
He added: "All the main economic and security posts in Government agree it would be hugely damaging to London to leave Europe."
Liberal Democrat leader Tim Farron has passionately pushed for Britain to remain in Europe since inheriting the position of his equally pro-EU predecessor Nick Clegg.
Mr Farron also colourfully attacked Boris Johnson for putting his personal ambition ahead of the national interest, claiming the London Mayor had had "more positions on Europe than the Kama Sutra".
London Mayor Boris Johnson became the highest profile figure in the Leave campaign - with "BoJo" being re-dubbed "BoGo" - after notifying the PM by text that he would be bidding for a Brexit.
Ten minutes later he was explaining his reasons in front of a large media throng outside his home, though denying it was part of a calculated long-term bid for Tory leadership.
"This is a once-in-a-lifetime chance to vote for real change in Britain’s relations with Europe," Mr Johnson said.
Justice Secretary Michael Gove - a long-time close ally to David Cameron and George Osborne - defied both by declaring his support for an exit.
Mr Gove described his preference as "the most difficult decision of my political life" and said it "pained" him to disagree with Mr Cameron.
But he said Britain would be "freer, fairer and better off outside the EU" and said an exit would "show the rest of Europe the way to flourish" in isolation.
Work and Pensions Secretary Iain Duncan Smith has claimed leaving the EU would give Britain greater protection from Paris-style terror attacks.
He told ITV News Political Editor Robert Peston it would prove easier to protect Britain's borders and stop unwanted people entering the country if Britain was outside the EU.
"Whether or not it stops a terrorist attack I can't predict that at all, I can only say controlling your own borders gives you that greater capacity," he said.
Leader of the House of Commons Chris Grayling has said the EU is holding Britain back.
"We can't control our own borders," he has said. "We can't set a number whereby people can come live and work here (and) we can't forge trade agreements ... without going through the European Union".
Mr Grayling told Peston he and other ministers would accept the "will of the people" and move on if the public voted for Britain to stay in the EU.
Culture Secretary John Whittingdale was another to join the Leave campaign, though he has insisted his party leader's future is not on the line.
Mr Whittingale accepted a £1,000 charity bet with presenter Piers Morgan on ITV's Good Morning Britain that David Cameron will remain Prime Minister even if the British people go against his wishes in June's referendum and vote for Britain to exit the EU.
Northern Ireland Secretary Theresa Villiers has said it is the "safer option" for Britain to withdraw from the EU.
While accepting there are valid arguments on both sides of the debate, she said an EU exit meant the chance to "take back control of our country".
Employment Minister Priti Patel said Britain's exit from the EU would see it become a "genuinely sovereign country" and reject the "unaccountable federalist European agenda".
She suggested people across the country were on the side of the out campaign, saying: "The British public want less Europe and more Britain."
Armed Forces minister Penny Mordaunt added her name to the out campaign in the wake of Boris Johnson's announcement, pitting her directly against her department superior Michael Fallon.
Likewise, Foreign Office minister James Duddridge has stood apart from Foreign Secretary Philip Hammond by offering support for an exit.
Unsurprisingly for the leader of a party founded on the commitment to leave the EU, Ukip's Nigel Farage is fighting for an immediate exit.
He said the PM's renegotiations were "marginal" and had left the remain campaign in a "rather weak position".
Mr Farage said UK voters in the June referendum would base their decision on topics such as the ability to veto EU law and not "marginal" issues like migrant benefits.
Conservative London mayoral candidate Zac Goldsmith praised David Cameron for his "efforts" at reforming Britain's relationship with the EU but said "the deal on the table does not go far enough".
"Given that even in the face of the UK voting to leave, the EU is unwilling to entertain meaningful reform, it is unlikely we will see anything more in the years to come," he said.
ITV News Political Editor Robert Peston noted that Mr Cameron is presumably "furious" at the decision after helping the Richmond Park MP with one of the key issues in the London campaign.
Goldsmith's stance places him alongside fellow London mayoral candidate, the Respect Party leader George Galloway, in advocating a departure.
Galloway has already formed an unlikely alliance with Ukip leader Farage as part of the cross-party Grassroots Out campaign.
"We are not pals," Galloway has said. "We are allies in one cause. Like Churchill and Stalin."