The PM blasts back

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Media captionTheresa May: "We should be working together, not pulling apart"

This is turning out to be quite a week.

Not just because of the government's biggest climbdown so far over its first proper Budget - but the constitutional clash that is already running at 100mph, having only begun in earnest on Monday!

Nicola Sturgeon dramatically threw the gauntlet down to Theresa May at the start of the week. Then Mrs May seemed reluctant to pick it up. But today, having considered her options, the PM blasted back at the first minister with a phrase that I suspect might stick: "Now is not the time."

Bump in polls?

That has, predictably, led to an explosion of criticism from the SNP - warnings to the PM that she will "rue the day" and suggestions from the first minister that this could be the day that "sealed the fate of the union".

It is extremely likely, as one senior unionist figure in Scotland told me today, that the Downing Street move is likely to give a "bump" to the independence cause in the next set of polls, at least temporarily.

SNP sources claim this means the Westminster government is essentially blocking an independence referendum indefinitely. What's actually happened is that Theresa May has ruled out discussions about a vote for two years.

It would certainly be a rush for the Scottish government to be able to hold another referendum before their next set of elections in 2021, but that is not the same as saying that it will never again be allowed to take place.

But for many attracted to the idea of Scotland going it alone, and also for many whose views are not settled, the perception that a Conservative prime minister is telling Scotland what it can and cannot do could be toxic.

So what on earth was Theresa May thinking? Here's Number 10's calculation. A majority of Scots chose to stick with the union the last time round.

'Only reasonable choice'

I'm told months of private Tory focus groups this year with voters have suggested their message of "not now" will be a relief to people on both sides of the argument who simply don't want to go through it again.

For many Scots voters the referendum was an exciting, and energising, political moment.

For others it was upsetting, and divisive, and they just don't want a return.

But this is extremely dangerous territory for any British government, not least a Tory administration with only one MP in Scotland, and all the tense history between the two centres of gravity.

Technically, legally, it is down to Westminster to permit another vote. And UK government sources suggest today's action was the only "reasonable choice".

Ruling out a referendum for good was enormously dangerous. Saying yes would have run counter to every instinct in Number 10.

But telling the Scottish government they won't even discuss another referendum until we leave the EU? It's an enormous risk.