It is said that brevity is a virtue. If it is, Boris Johnson's letter to Nicola Sturgeon refusing Westminster consent for a second independence referendum shows the Prime Minister to be a paragon of that virtue.

His 'Dear Nicola' missive to the First Minister is short, sweet and to the point. It takes Mr Johnson just a few short sentences to say 'No'. It is not a detailed rebuttal of the Scottish government's case.

In the eyes of the new Tory government in Whitehall this is a master stroke. No messing around, no diplomatic verbiage, no prevarication. Nicola isn't getting IndyRef2. End of.

That was the message today from the Chancellor the Duchy of Lancaster, Michael Gove, today. He did not equivocate.

To the SNP government this brevity demonstrates Mr Johnson is a politician of easy political virtue, prepared to play fast and loose with a referendum mandate they claim from the last UK election.

His easy dismissal of their case is, in the eyes of Ms Sturgeonand her ministers, symptomatic of haughty Tory disdain for Scotland and the nation's right to choose its constitutional future.

After all, the Scottish government published a 38 page document 'Scotland's right to choose' setting out their case. And it was met with what they see as an insultingly short response.

The reality is somewhere in the middle. The Scottish document, for example, has a four page Scottish history timeline in it. Fascinating, but not perhaps strictly relevant to modern constitutional debate?

And although the Prime Minister's reply was brief, there has been a detailed process across Whitehall looking at the SNP's claims, not least to make sure the UK is on firm legal ground.

Credit: PA

Yet for all the outrage from the Scottish government, and all the firm talk from the UK government, both sides always knew what the answer was going to be from Mr Johnson.

What matters more is where, if anywhere, the SNP's promise to hold a second referendum goes from here, and how the UK government respond. That's Ms Sturgeon's dilemma.

I've spoken today to Michael Russell, the Scottish government's cabinet secretary for constitutional relations, and he was adamant there will be a referendum this year.

I pressed him on the detail and all he would say was that in a matter of weeks the First Minister would set how she intends to take forward the referendum.

Now, there may be some way this can happen. It is possible that the Scottish government could take the UK government to the Supreme Court to test the legislation.

However, most constitutional and legal experts believe that, like it or not - Unionist do, nationalists don't - the power over matters like referendums lies with Westminster.

Which should take us from courts and constitutions to politics, electoral politics. Assuming a referendum does not take place this year, it will become a major issue in the 2021 Holyrood election.

For the nationalists, the argument will - once again - be Scotland's 'right to choose', the democratic case for an independence referendum, which they hope to win.

If the SNP win an outright Holyrood majority, difficult with the proportional voting system, their case would be hugely strengthened. It would be much harder for Westminster to continue to say 'no'.

So the key objective of the opposition parties, and specifically the Tories as the main opposition party, will be to stop that happening. For them, stopping an SNP win stops IndyRef2.

And to achieve that, expect relentless scrutiny of the SNP domestic record on issues like education, health, justice and the economy - all areas where they have questions to answer after 12 years in office.

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