TIME is pressing as the constitutional game of three-dimensional chess between London and Edinburgh moves towards a possible conclusion.

Theresa May, recovering from the post-Budget turbulence of the NICs row, is preparing for her big move: triggering Article 50.

The Brexit Bill returns to the Commons on Monday. If, as the Prime Minister hopes, MPs overturn the changes made to it by the House of Lords and peers decide to end their ping-pongery, then the legislation could be given Royal Assent quickly; even as early as Tuesday.

Normally, Mrs May would give her EU summit statement to the Commons on Monday but this has been put back a day because she will be attending a church service to celebrate the Commonwealth.

Of course, it could just be a happy coincidence that the PM has left the political limelight on Monday to fall squarely on the Bill’s progress and so on Tuesday, when she addresses MPs on matters EU, she could announce her Article 50 email is already in Donald Tusk, the European Council President’s inbox.

Officially, the line remains that Mrs May will trigger Article 50 by the end of this month; so technically she could leave it until Friday March 31.

Any time around Saturday March 25 is a no-no as there will be celebrations to mark the 60th anniversary of the founding of the EU; the PM will want to avoid raining on the EU’s parade.

Equally, next Wednesday, the Dutch go to the polls. Could any publicity about the start of Britain’s withdrawal give a boost to Geert Wilders’s Freedom Party, which is campaigning for Nexit?

Then there is the little matter of the SNP’s spring conference in Aberdeen next Friday and Saturday.

Triggering Article 50 just two or three days ahead of the Natfest with the prospect that Mrs May’s letter will contain nothing to satisfy the First Minister that “Scotland’s voice” has been listened to could crank up the atmosphere to fever pitch.

Last week, Angus Robertson, the deputy SNP leader, made clear if the UK Government could not reach agreement with Edinburgh to protect Scotland’s interests, “there will be another referendum, yes".

This week Ms Sturgeon put a timeframe on indyref2, saying autumn 2018 was a “common sense” choice.

Despite a poll suggesting the independence race is now neck and neck between those for and those against, there is a rumbling unease among some Nationalists that the “mood of Scotland” is nowhere near strong enough yet to demand another poll.

With her government consumed by Brexit, Mrs May does not want to face a second Scottish referendum; not least because if she lost, there would be parallel divorce proceedings with Edinburgh.

Sir Michael Fallon, the Defence Secretary, recently made public what some ministers have said in private: the UK Government would not facilitate indyref2 in this parliament.

Of course, this could be just what a conflicted Nicola Sturgeon wants as it would unleash a political backlash against Westminster and, most crucially, buy her time to get those opinion polls up.

Both the PM and FM will be hoping for a constitutional checkmate but as the uncertainties around Brexit seem to multiply by the day we could just end up with an ongoing stalemate.