The cabinet minister in charge of Scotland has issued a stark warning that the country could sleepwalk into a split from the UK because unmotivated unionists are failing to wake up to the threat posed by Alex Salmond's nationalists.

Amid clear signs of tension and division in the no campaign, the Scottish secretary, Alistair Carmichael, said he believed the nationalists had greater "hunger" for victory and could create an unstoppable momentum.

Carmichael, a Liberal Democrat, said he believed the yes campaign, which he said was possibly the best financed in British political history, could well move ahead in the polls during the runup to the 18 September referendum.

In an interview with the Observer, he expressed deep concern that some supporters of the union were assuming victory and failing to shout loudly enough in favour of remaining part of the UK. "The danger is that by the time they realise it could happen, it could be too late," he said. "Everybody needs to know that this is a serious contest, and one which it is not impossible that the nationalists could win."

Carmichael said the no campaign should learn from the nationalists' direct, sometimes aggressive, approach: "We're never going to match them for the spend, but in terms of the hunger I think we have to match them for just how badly we want this. That is always going to be a challenge, because for nationalists this is the issue that defines them, whereas for a Labour supporter, a Liberal or a Conservative, this can be an issue you care about but is not one that defines you. So that is where we need to work harder at motivating our people in a way that their people come ready motivated."

The SNP and the yes campaign were in fighting mood as the chancellor, George Osborne, and the chief secretary to the Treasury, Danny Alexander, tried to quash suggestions that the UK would, after all, agree to form a currency union with an independent Scotland. The two Treasury ministers said: "Walking out of the UK means walking out of the UK pound. A currency union will not work because it would not be in Scotland's interests and not in the UK's interests."

Their denials were issued after a minister involved in the no campaign privately told the Guardian that despite public statements to the contrary, a currency union would have to be formed to ensure financial and economic stability in the event of independence.

The recent official line of the no campaign has been that a currency union would be impossible as it would not be in the interests of either Scotland or what remained of the UK. The unnamed minister's comments were seized on by deputy first minister Nicola Sturgeon.

"This was supposed to be the no campaign's trump card, but it has backfired badly. The gap between yes and no has halved since November, and most Scots simply do not believe the bluff and bluster we've had from George Osborne, Ed Balls and Danny Alexander," she said. "Now that the card has been withdrawn, it gives an even bigger boost to the yes campaign and can only add to the sense of crisis engulfing the no campaign."

While Carmichael did not criticise the Better Together campaign, led by former Labour chancellor Alistair Darling, he made clear he wanted it to be more proactive, with a greater range of voices, and to turn up the volume. "You have in Alistair Darling a first-rate campaign head, but I would want to hear and would expect to hear a wider range of Labour voices coming into the debate as it gets closer to polling day. The interventions of Gordon Brown have all been good and positive and helpful, and I'd like to see more of that."

While recent polls have shown Better Together still ahead, one recent survey, by Panelbase, put support for independence at 40%, its highest level since campaigning began in earnest and just five points behind the pro-union side. The undecideds were on 15%.

Better Together agrees with the Scottish secretary that the polls could narrow further, but says it remains confident of victory. A spokesman said it would be focusing on the one million people out of an electorate of four million whose votes are "still up for grabs".

Carmichael said he was "hopeful" of victory but could not rule out a nationalist win: "It is not impossible. I am not expecting to lose, but it is eminently possible that they will be able to buy momentum with the advertising and campaign resource they have. If they do, it could all get very difficult." Asked if the no campaign was good enough at modern campaigning, he said: "I think we are, but we haven't yet got the volume of it. The basic messages are the right ones. They are delivered in a fairly professional way. We just need to do more of it."

Asked whether the no campaign had been too negative, he said that might have been the impression on occasions, because the nationalists framed everything as negative: "The arguments we have are positive, but we have allowed ourselves to be defined by the other side as negative."The fascinating thing is that the nationalists come out with some of the most incredibly negative material. You had Alec Salmond coming here talking about London being the dark star. He was talking about Scotland's oil being stolen by the thieves at Westminster."