No planning is being done by the Treasury for the consequences of Britain voting to leave the European Union, George Osborne has said, as he expressed growing confidence of a renegotiation deal.
The Chancellor said the "essential pieces" of Prime Minister David Cameron's push to secure a reformed relationship to persuade voters to back continued membership were falling into place.
Speaking after talks in Berlin with counterpart Wolfgang Shauble, he said Germany and France were among an emerging consensus that Britain was making a "perfectly reasonable case for change" that could benefit the whole bloc.
He dismissed the prospect of a second vote, insisting the referendum would settle the question of Britain's membership for at least a generation and probably for his lifetime.
And he suggested pro-"Brexit" campaigners would have difficult questions to answer in the campaign.
Mr Cameron is seeking to secure final agreement at February's summit before calling an in/out vote - but still faces a challenge persuading some countries to accept welfare curbs on EU migrants.
The looming campaign has begun to publicly expose Conservative divisions, with Commons leader Chris Grayling giving what was seen as a clear signal he intended to back Brexit.
Asked if Treasury officials were drawing up plans for Britain leaving the EU, Mr Osborne told BBC2's Newsnight: "No, the Treasury is 100% now focused on achieving the renegotiation.
"That is where the resources of the Treasury are deployed."
Mr Osborne joined the Prime Minister in insisting he had not ruled out campaigning to leave the EU if a deal could not be struck - but said no plans were being made for that eventuality.
"Our focus, our efforts, are on making sure that we achieve a successful renegotiation and I see, not just here in Germany, but in France, other key member states in the European Commission, a consensus emerging that Britain has made a perfectly reasonable case for change, that this new settlement we're offering is not just better for Britain, but potentially better for the rest of Europe as well.
"So I am optimistic.
"Just a few weeks before a crucial European Council where we're going make these decisions ... I see the essential pieces of the deal falling into place.
London mayor Boris Johnson is among those who have suggested that a public endorsement of Brexit could prompt fresh negotiations on an improved deal to put to the public.
"There's no second vote. This is the crucial decision of our lifetimes," Mr Osborne said.
"I think anyone who votes out on the assumption that a year or two later you can have another vote to vote back in is being unrealistic about the nature of the choice and I think it's really important that the British people focus on the fact this is the once in a lifetime decision.
And he said those arguing against membership still had to answer a number of questions.
"If it comes to the referendum and there are people advocating that we leave in that referendum, they are going to have to answer the question: what is the alternative, are we going to have free movement of people, are we going to have to pay into the European budget in order to have access to their market?" he said.
"Are we going to have to sign up to their rules, even if we don't have a vote on those rules? Those are all the things that countries like Norway face today, and they're going to be good questions to put during the referendum campaign."
Mr Osborne said he considered himself a Eurosceptic but it was "perfectly respectable" to have criticisms and still campaign to stay in a reformed EU.
"I'm a Eurosceptic like many of my Conservative colleagues because I've been concerned about some of the things that have happened in the European Union, that's why I want to make those changes," he said.
"It's a perfectly respectable position to say, let's seek those changes, let's achieve those changes, let's have that new settlement, and then we can have the best of both worlds: we can be in the European Union, but not run by the European Union, to use an old and very apt slogan.
"I think the majority (of voters) want to stay in a reformed European Union and that's why this renegotiation matters because it offers the chance of a new settlement between Britain and Europe, where we're not part of ever closer union, where the eurozone can't impose changes on us, they need our consent, and I think if we achieve that new settlement, then we will finally have put at ease that often fractious relationship between Britain and Europe."