HEELS are being firmly dug in on either side of the border.

After Nicola Sturgeon insisted at the weekend she will not budge on a “matter of principle” over the UK Government’s Brexit Bill, it now appears to be Theresa May’s turn to make clear she is not for turning.

The nub of the matter is about where power lies.

The First Minister believes all 111 powers and responsibilities returning from Brussels post-Brexit should not be parked at Westminster but sent directly to Holyrood; that, after all, is what the devolved settlement is all about.

But the Prime Minister insists her responsibility is to look after the well-being of the whole of the UK, including the integrity of its single market.

Her argument goes that if all the powers went to Edinburgh, then Ms Sturgeon and her colleagues could, in theory, begin swiftly to introduce changes to matters like fishing, agriculture and environmental protection, which could undermine the free-flow of the UK’s single market.

UK Government insiders say it is not that all the powers won’t eventually go to Holyrood but that there will simply be a temporary freeze on 25, so that the harmonised frameworks can be agreed.

But the FM smells a rat; that in forming those frameworks Whitehall wants to grab control of powers that should be exercised fully and without restriction by the Scottish Parliament.

The row centres on the passage of the UK Government’s EU Withdrawal Bill. While they do not accept the “power-grab” argument, Tory ministers have agreed nonetheless to change Clause 11, dealing with devolution, to allay concerns; including those from their own side.

But unless Whitehall agrees to move further and hand over all 111 powers and responsibilities to Holyrood, then a majority of MSPs will not give their consent.

The PM could have until May to change the bill in the House of Lords but if the FM remains immovable, then the country will be facing a constitutional crisis.

Mrs May and Ms Sturgeon meet next Wednesday for a Downing Street showdown. The question is: will either of them blink?