If UK cabinet ministers were in any doubt about the difference in style between Alistair Carmichael, Scotland secretary, and his unflappable predecessor, Michael Moore, it dissipated after the new man gave his first cabinet briefing.

Mr Carmichael, the former Liberal Democrat chief whip who has been brought in to add passion to the independence debate, told colleagues starkly on Tuesday they could be heading for defeat in next year’s referendum.

Despite a clear and consistent poll lead for the pro-union campaign, he warned that many voters remain undecided. He added that the poll lead could vanish as quickly as Labour’s did during the final weeks of the 2011 Holyrood election, a dramatic reversal that granted the SNP an unprecedented majority.

“If there was any complacency among cabinet members beforehand, by the time I left the room, it had gone,” he said.

With the historic referendum less than a year away, pro-union campaigners are preparing for next week’s publication of the Scottish government’s much-delayed independence white paper.

The paper is the Scottish National party’s last big chance to answer many of the questions it has been accused of dodging, on everything from the currency of an independent Scotland to its membership of the EU and border arrangements.

Pro-union leaders want to focus the debate on three areas: currency, pensions and the deficit of a separate Scotland, believing they can win the argument on all three.

“[The SNP] want to keep that focus on the heart. We need to keep the debate on the territory of the head,” said Mr Carmichael.

They have been helped in the past week by two reports from unaffiliated institutions. The first, by the Institute for Fiscal Studies, warned that Scotland would have to add £6bn to its deficit if it voted for independence, thanks to declining oil reserves and an ageing population.

The second, a report from the National Association of Pension Funds published on Thursday, said unpicking cross-border pension schemes would be costly and complex, and could lead to some defined-benefit schemes having to close.

Meanwhile, Number 10 added another argument into the mix on Thursday, arguing that Scotland would lose its share of the UK budget rebate if it tried to rejoin the EU as an independent country.

Mr Carmichael is also keen to counteract the more emotional SNP argument that a vote for separation is a patriotic one. With a flash of anger, he says: “I don’t want my half-English children to be growing up in a foreign country to the one where their grandparents live. Questioning people’s patriotism because of their view on what is the best constitutional arrangement crosses a line.”

Mr Carmichael is conscious, however, that victory may rest with a few thousand wavering voters in western Scotland. They normally vote for Labour, but in 2011 switched to the SNP. These, he says, are largely white males aged 22 to 50 who feel disillusioned with much of British politics.

He said: “There is still a big chunk of undecided voters who are people who went almost wholesale to the SNP in 2011. If they did that next year, the polls would narrow to the point where losing would be entirely possible. We take nothing for granted.”

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