The UK will vote on whether to remain in the EU on Thursday 23 June, Prime Minister David Cameron has said.
The prime minister made his historic announcement in Downing Street after briefing the cabinet.
He said he would be campaigning to remain in a reformed EU – and described the vote as one of the biggest decisions “in our lifetimes”.
Ministers immediately divided up into the leave and remain camps as the campaigns got under way in earnest.
Home Secretary Theresa May heads the list of those who announced they will campaign to stay – but Justice Secretary Michael Gove has signed up to the leave campaign.
London Mayor Boris Johnson, who was not at the cabinet meeting, has yet to declare where he stands.
In his statement, Mr Cameron warned that leaving the EU would be a “leap in the dark” as he urged voters to back his reform deal.
“The choice is in your hands – but my recommendation is clear. I believe that Britain will be safer, stronger and better off by remaining in a reformed European Union.”
Mrs May said the EU was far from perfect but “for reasons of security, protection against crime and terrorism, trade with Europe, and access to markets around the world” it was in the national interest to remain in.
Mr Gove said it had been the most difficult decision of his career to go against the prime minister but he believed “our country would be freer, fairer and better off outside the EU”. He added: “Far from providing security in an uncertain world, the EU’s policies have become a source of instability and insecurity.”
Commons leader Chris Grayling, another leave campaign backer, told the BBC: “I actually believe the EU is holding this country back. We cannot control our borders, limit the number of people who come here do trade deals.
“I do not believe we can take decisions in the national interest when we are part of the European Union.”
He said the prime minister had “put in a Herculean effort to try to deliver change” to Britain’s relationship with the EU, but the “concessions” he had brought back from Brussels did not “give us the opportunity to take decisions in the national interest” without consulting Brussels.
Mr Cameron claims his EU reform deal – hammered out at a two-day summit in Brussels – will give the Britain “special status” within the bloc – tackling concerns over migrants getting “something for nothing” from the benefit system and exempting the country from the EU drive for “ever-closer union”.
But critics say it does nothing to tackle high levels of immigration or take back powers from Brussels, with UKIP leader Nigel Farage branding it “truly pathetic”.
Britain Stronger in Europe released a campaign video accusing leave campaigners of “utter hypocrisy” for attacking Mr Cameron’s deal, claiming that many of them have been calling for the same reforms.
Alan Johnson, chairman of Labour In for Britain, said the party would be “united in making the case that Britain is better off in Europe”.
“Our EU membership brings Britain jobs, investment, trade and keeps us safe. Leaving the EU would put all that at risk and diminish Britain’s influence in the world. It was pressure from Labour that meant David Cameron has been prevented from trading away rights at work in this deal and we can now get on with our campaign to keep Britain in Europe.”
Scotland’s First Minister Nicola Sturgeon said in a statement the SNP will be “leading the positive case to keep Scotland in the EU”.