Prime Minister David Cameron on Sunday begins the daunting challenge of persuading his country to stay in the European Union, after announcing a June 23 referendum on its membership of the bloc.
The conservative premier will face the outspoken leader of the eurosceptic UK Independence Party (UKIP), Nigel Farage, and Scottish First Minister Nicola Sturgeon on the BBC's Andrew Marr show from 0900 GMT.
Cameron announced the date for the referendum on Saturday after a two-hour cabinet meeting where he briefed ministers on a deal struck in Brussels that he said will give Britain "special status" in the EU.
But, with the ruling Conservative Party deeply divided over Britain's place in Europe, six members of his 22-strong team including justice minister Michael Gove had already announced by the end of the day they would campaign to leave.
"We are approaching one of the biggest decisions this country will face in our lifetimes," Cameron said, addressing the nation outside his 10 Downing Street residence.
"The choice goes to the kind of country we want to be," he said, warning that proponents of leaving were offering "a risk at a time of uncertainty, a leap in the dark".
Britain would be "safer, stronger and better off" in the 28-member bloc, he said, calling the concessions negotiated with other EU leaders "the best of both worlds".
The referendum campaign will be bitterly contested in a country with a long tradition of euroscepticism and a hostile right-wing press, with most opinion polls showing Britons are almost evenly divided.
Farage has branded the deal "pathetic," calling on the British public to back leaving the bloc: "The 23rd is our golden opportunity, let battle be joined."
Around 50 Conservative MPs had by Saturday come out in favour of leaving, while pro-EU MP Nick Herbert tweeted 100 Tory MPs would back Britain staying in -- almost half the 330 lawmakers the party has in parliament.
- 'Diverse' Europe -
A bigger blow for Cameron would be the loss of popular London mayor Boris Johnson to the eurosceptic camp, which has so far been plagued by in-fighting and has no clear leader. He is expected to reveal his decision on Monday.
The Mail on Sunday carried good news for Cameron with a Survation telephone poll showing the "in" campaign with a 15 percent lead, a huge swing from level previous surveys.
Cameron announced his intention to hold a referendum three years ago, under pressure from Conservative eurosceptics and the rise of UKIP.
He originally said he would only back being in the EU if he could secure reforms to address migration and competitiveness, among other issues, but after clinching the deal with Brussels said he will campaign "with all my heart and soul" to stay.
France's Foreign Minister Jean-Marc Ayrault also backed the deal in an interview published on Sunday, saying it "simply demonstrates Europe's diversity".
"It does not undo what has been done or hinder those who want to go further. The base principles and founding values of the EU remain," he told France's Journal du Dimanche.
But critics in Britain say the deal, which contained restrictions on welfare payments for EU migrants and an opt-out for Britain from the EU's goal towards ever-closer union, fell short.
"Call that a Deal, Dave?" headlined the Daily Mail, while The Daily Telegraph said Cameron had made "puny gains" and The Times called it "thin gruel".
Analysts say the nitty gritty of the EU deal will likely be only a small part of the referendum campaign, which is already focused on immigration, security, prosperity and Britain's place in the world.
Anand Menon, professor of European politics at King's College London, said he expected both sides to emphasise the risks of either leaving or sticking with the status quo.
"This is going to be a depressingly negative campaign," he told AFP.