David Cameron is to tell cabinet ministers they must quit the government if they want to campaign for Britain to leave the EU, as he prepares to face down hardline Conservative eurosceptics.
The prime minister expects to secure a new deal for Britain in a reformed EU and to fight for the UK to remain a member, throwing the government's full weight behind the campaign.
On Sunday, more than 50 Tory MPs formed a new group, Conservatives for Britain, warning Mr Cameron that they would fight for a "Brexit" unless he can "end the supremacy of EU law over more matters of British life".
Mr Cameron admits he cannot win over "irreconcilables" in his own party and will play tough with them, telling any ministers who want to campaign for a Brexit they must quit their government jobs.
The prime minister has told colleagues at the current G7 summit in Bavaria that all ministers will have to toe the official line.
Mr Cameron said that unlike Harold Wilson, who allowed Labour ministers to campaign on both sides of a Common Market referendum in 1975, he expected "everyone in government" to fight on the same side.
"I've been very clear that if you want to be part of the government, you have to take the view that we are engaged in an exercise of renegotiation, have a referendum and that will lead to a successful outcome," he said.
Meanwhile, Philip Hammond, the foreign secretary, dismissed demands by some Tory MPs in the new group for the Westminster parliament to have a unilateral veto over any EU law.
"That's not achievable, that's not negotiable, because that would effectively be the end of the European Union," he told the BBC. He said a group of national parliaments working together should be able to show a "red card".
However, the Tory MP Steve Baker, head of Conservatives for Britain, said it was a "modest demand that parliament should have sovereignty over its own territory". If that were not granted, Britain would "have to leave".
Mr Cameron will have his first clash with the new group this month when MPs debate amendments to the EU referendum bill, which paves the way for a British in-out vote by the end of 2017.
Tory eurosceptics could rebel against Mr Cameron for the first time since his May 7 election victory, over a provision that would allow the use of public money in the run-up to the vote to make the case for British EU membership.
The MPs want to see the normal 28-day "purdah" period of official neutrality before a sensitive vote. But Mr Hammond said the government had no intention of remaining "neutral" in the referendum campaign.
The Treasury is expected to publish a series of "analysis" papers looking at the economics of Britain's EU membership, which eurosceptics believe could see the official civil service being deployed in favour of the "in" campaign.
Owen Paterson, a former Tory minister, said: "The British state in cahoots with the Brussels machine will be able to fix the vote in its favour." Writing in the Mail on Sunday newspaper, he said this would be "totally unacceptable".
Graham Brady, chairman of the Tory backbench 1922 Committee, has argued that Mr Cameron could avoid a big confrontation by allowing ministers to campaign on both sides in the referendum campaign.
But Mr Cameron wants the whole government to mobilise behind a single cause. Although he has said he will campaign for a Brexit if he fails to secure any big concessions, the expectation in Downing Street is that he will lead the campaign for Britain to stay in.
President Barack Obama, also speaking at the G7 summit in Germany, said he was "looking forward" to Britain staying part of the EU to provide "leadership and strength on a whole host of global challenges".