Now David Cameron has secured his deal for a new relationship for Britain inside the European Union, cabinet ministers are free to campaign either to remain or leave.
Soon after the announcement that the referendum will be on 23 June, Justice Secretary Michael Gove became the first cabinet member to set himself against Cameron's call for the country to stay in the union, calling his move to back Brexit "the most difficult decision of my political life".
Gove said it "pained" him to oppose Cameron, but argued that the EU "prevents us being able to change huge swaths of law and stops us being able to choose who makes critical decisions which affect all our lives."
He was followed by others who also officially joined the 'Leave' campaign.
Johnson has yet to come off the fence
Downing Street is now reportedly waiting nervously for Mayor of London Boris Johnson to declare which side he will be on, in an announcement expected on Sunday.
Sources close to Johnson said he was still “genuinely torn” on the matter, The Guardian reported, and would continue to assess the offer of a new relationship with Europe from Cameron over the weekend.
Cameron said in his historic speech: "I do not love Brussels, I love Britain".
"We are Great Britain – we can achieve great things. That is not the question in this referendum," he said. "The question is will we be safer, stronger and better off working together in a reformed Europe or out on our own. I believe we will be safer in a reformed Europe, because we can work with our European partners to fight cross border crime and terrorism.
"I believe Britain will be stronger in a reformed Europe because we can play a leading role in one of the world’s largest organisations from within, helping to make the big decisions on trade and security that determine our future.
"And I believe we will be better off in a reformed Europe because British businesses will have full access to the free trade single market, bringing jobs, investment and lower prices. Let me be clear. Leaving Europe would threaten our economic and our national security."
Organisers of Glastonbury festival also called on the event's attendees to sign up for a postal vote in the referendum after it was revealed the date clashes with the five-day music event.
Though some cabinet members expected to back Brexit have declared support for Britain staying in the EU this weekend, not all of Cameron's top team are on side.
Here's the list so far of who is 'in' and who is 'out':