The prime minister was paying his first campaigning visit to Scotland since his intervention last month to speed up the referendum process and used the trip both to meet Salmond and to proclaim his ‘head, heart and soul’ belief in the benefits of union. He called the preservation of the UK ‘my absolute priority’.

Cameron continued to insist that the referendum should be a simple ‘yes-no’ choice on independence, rather than offer other options like fiscal autonomy within a federal UK, often called ‘devo-max’. But he also indicated that more devolution would follow if Scots voted to retain the Union.

‘We have to settle that question before then going on and asking, I think quite legitimately, is there more that we can do to improve the devolved settlement?’ Cameron said.

‘Are there powers that could be devolved, and how can we make the United Kingdom work better?’

Asked if this meant that he was prepared to extend devolution, he said: ‘I'm very prepared. I believe in devolution, and I don't just mean devolution in terms of power, I mean devolution in terms of giving people greater control over their own lives.

‘What I’m offering is that if the answer to the question is that Scotland wants to stay in the UK, further options on devolution are on the table and we can discuss these.’

Cameron also said he accepted that Scotland was capable of survival as an independent nation: ‘My argument is, we are better off, we are stronger together, we're fairer together, we're richer together,’ he said.

But Salmond was unimpressed by Cameron’s arguments, and evoked memories of Lord Home’s infamous 1979 promise that the Conservatives would deliver a better devolution scheme if Scotland voted to reject the devolution scheme then on offer.  He challenged Cameron to make clear without delay what extra powers he had in mind, so that they could become part of the debate straight away.

He said Cameron was 'on very, very shaky ground' by urging Scots to vote no on the vague promise a better devolution offer, and doubted they would not fall for it again.

'If the prime minister has an offer to make to the people of Scotland then he should make it now, so that there can be a clear debate and a clear decision,' Salmond added.