Surprise as Moore says that he is not a 'Unionist'

They also place him at odds with many of his cabinet colleagues, including Prime Minister David Cameron, who has repeatedly described himself as a proud Unionist.

However, the Liberal Democrat minister, whose job is to represent Scotland's interests in the UK, specifically rejected the label.

"I don't use that term to describe myself," he said in an interview with a political magazine.

Although he made clear that he opposed independence, he claimed every UK party had accepted the "old-fashioned centuries-old version of the UK is horribly outdated".

Last night the SNP seized on his comments and claimed the Scottish Secretary's constitutional position had crumbled into a "mess of confusion and contradiction".

Mr Moore is not the first leading politician from an anti-independence party to disown the tag.

Labour leadership contender Ken Macintosh has also said he is not a Unionist.

He added that, if he wins this month's election contest, he will instead lead the party as a "devolutionist".

But Mr Moore's intervention will be seen as highly significant because of his role in formulating the UK Government's response to the independence referendum.

The timing is also difficult, coming as the Unionist parties continue informal talks on how to oppose the SNP's separation campaign.

Mr Moore told Total Politics magazine: "Every one of the UK-wide parties, whatever journey they've been on, has recognised that the old-fashioned, centuries-old version of the UK is horribly outdated and we need to develop it."

Wales was currently considering "what its future might look like", he said, while "England, over time, will find its own voice and how it wants to see that develop, too".

"Devolution is where it's at," he added, "and nobody is thinking in terms of old-fashioned unionism any more."

His comments come just weeks after his party announced a commission on home rule headed by the party's former leader Sir Menzies Campbell.

Mr Moore suggested in the interview that the group would propose devolving more powers to Scotland,

However, he said he would want to build "consensus" rather than demand immediate change. He also hinted he might be the last ever Scottish Secretary.

In future the role might be amalgamated with the Wales and Northern Irish jobs to create a Secretary "of the nations", he said.

But the SNP's chief whip, Stewart Hosie, said: "Michael Moore's position on where he stands constitutionally has crumbled into a mess of confusion and contradiction.

"He says he wants home rule, yet opposes the option of taking up a devo max proposal in the referendum.

"If he genuinely supports home rule and the devolution of further powers to the Scottish Parliament he has the opportunity presented by the Scotland Bill to work with the SNP Government and deliver more powers for the Scottish Parliament."

He added: "The LibDems' only contribution to constitutional change is to obsess about the SNP and block every proposal put on the table, even the ones they claim to agree with."

A spokesman for Mr Moore said that he did not mean he longer believed in the union between Scotland and England, but was no longer content with the way that the term Unionist was often understood.

"What he was saying was that he was not a static Unionist in that way. He was talking about being a federalist," he said.

He added: "He supports the UK and he is a committed believer in devolution."

Mr Moore has previously faced attacks from the other Unionist parties that he was too slow to react when May's unprecedented Holyrood majority guaranteed that the SNP could hold a referendum.

In recent months, he has repeatedly attacked the SNP's plans to hold a vote on breaking away from the UK and pressed Scottish ministers on a series of questions over what leaving the UK would mean for Scotland.