Ruth Davidson is right.

The future of the United Kingdom as a unitary state is not secure.

She was also right to go to London earlier this week to deliver that message. That is where it needs to be heard. I know no one in Scotland who thinks that last September's referendum settled the question. The few who might have done were brutally disabused of that notion in the early hours of Friday, May 8.

In fact, the message needs to be heard not in Dover House, where it has long been understood, but rather in 10 Downing Street. It is from there the biggest threat to the Union now comes.

At 7am in the morning of September 19, following the referendum result, the Prime Minister emerged to thank the people of Scotland for sticking with one of the most successful political unions the world has ever seen and to reaffirm his commitment to its future. This was his time to tell the people of a continuing UK that he understood what had taken us to the brink and that he would deliver a modern UK, fit for the 21st century.

Except, he didn't. Instead, in one of the most egregious pieces of self-serving politics ever seen (and remember, Downing Street is a high bar to clear) he spoke to his own backbenchers and party activists who were increasingly spooked by the referendum on the one hand and Ukip on the other.

For Mr Cameron, the threat of September 18 had passed and the focus, with an almost psychopathic ruthlessness, switched to the General Election on May 7. Dave the Unionist was dead; long live Dave the English Nationalist.

And so it continued through to close of polls. There is no denying that it worked.

Faced with a nationalist tide in Scotland, Mr Cameron was able to surf a wave of nationalist sentiment in England that eventually carried him far enough up the beach to secure a majority in the House of Commons that few had predicted.

As a result, we have nationalist governments in Holyrood (the SNP) and in Westminster (the Conservatives).

That is why the Union remains in peril. Can it be saved and, if so, what can save it? Of course, it can be saved but, at present, almost everything the UK Government does makes this look less than certain.

I believe the Prime Minister is sincere when he says that he wants to maintain the Union. I am less certain that he understands what it will take to achieve that end. The answer no longer has anything to do with what power or which taxes can be exercised by the Scottish Parliament. That merely plays the nationalists' game. The answer lies in bold constitutional reform across the whole of the UK. It lies in a federal structure where the people of England can be empowered by devolved government in the same way in which the people of Scotland, Wales and Northern Ireland have been.

It lies in giving the people of England the power to decide exactly what devolution means for them. Is it an English Parliament or a structure of regional assemblies? Only the people of England can decide that.

It lies in a written constitution, a parliament that reflects and respects the views of all who vote for it and where everyone has their say through a democratic mandate and not political patronage or the hereditary principle.

It lies with a UK-wide constitutional convention, where people beyond the political and media classes can have their say and shape their destiny.

It does not lie in a dangerous fudge of English votes for English laws that allows conservatives (in any party) to continue to pretend that, somehow, constitutional change can be accommodated in an institution that ceased to function as a representative democracy in 1974.

That is the logical conclusion of Ruth Davidson's slightly breathless assertion that the Union remains in peril. Will she, I wonder, have the courage to speak truth unto power and take that message to her boss in No 10?

Will he, in turn, have the courage to do the necessary to redefine the Union in a way fit for the 21st century?

This is a defining moment for Mr Cameron. Will he stick with the agenda of the English nationalists and risk the Union or will he embrace democratic renewal and cement it?

Alistair Carmichael is Liberal Democrat MP for Orkney and Shetland. He was Secretary of State for Scotland from 2013 to 2015.