By Alex Stevenson

A Scottish independence referendum could be blocked in the courts before it even happens, London is warning. The slow ratcheting-up of its brinksmanship is designed to show Holyrood that the Scotland Office means business.

After last week's opening of hostilities over the terms of the Scottish independence referendum – and, crucially, who will get to determine them – the UK government is now hardening its approach.

Advocate-general William Wallace is due to make a speech on the issue on Friday but was this lunchtime rolled out earlier than planned to get his point across. He repeated the UK government's position that it makes no difference whether a referendum held under the authority of the Scottish government is 'consultative' or 'binding' – either way, he insists, "the Scottish parliament has now power to legislation for a referendum on independence".

Last week the Scotland Office's lawyers were cautious, behind the scenes, about whether it would be possible to block such a referendum from taking place. They seemed to think it probably would be, in line with the convention

But conventions are made to be broken, and there was always a risk that Alex Salmond and co might decide to ignore that particular rule on this occasion. Certainly his defiant behaviour over the last few days has underlined the risk. Which is why we now see today's rhetoric hardening up. Lord Wallace of Tankerness has made the position much clearer:

"Scottish parliament legislation paving the way for an unlawful referendum would be open to challenge in the courts," he says.

"That challenge could be initiated in the court of session by any individual or organisation with sufficient interest to bring proceedings.

"It is important to note that a successful challenge in the courts could not only invalidate the outcome of the referendum – it could stop the referendum happening in the first place."

The ball is, once again, in the nationalists' court. I suspect they will be happier than might initially be expected at this latest turn of events. For this battle is about the rules of the game rather than the actual debate itself, and so – even thought the SNP are as keen to win this fight as the three mainstream Westminster parties – it is obviously better for them to be seen responding to aggression from London, rather than making the initial play themselves.

In short, they will be hoping the aggression on show from London wins them more political support for independence, rather than the other way around.

When I first noted that the UK government could block the referendum in the courts the reaction was hostile to London in the extreme. One tweeter described the move as a "North Korean solution to pesky democracy". Another wondered whether this was "like a husband going to court to prevent his wife from divorcing him?"

Brazen meddling with the rules of the game: that's how people are interpreting the UK government's manoeuvres at the start of what is set to be a second acrimonious week. No wonder, then, that it looks like they'd rather there was no independence referendum – unless on their terms – at all.

The question then becomes: will they lose more political capital in insisting they get their way than they will gain in ensuring the referendum only has two options, and takes place sooner rather than later? It's far from clear, I think. Please do let me know your thoughts on Twitter at @alex__stevenson or drop me a line at alex.stevenson@politics.co.uk…