THE Scottish and Welsh governments have launched a pincer movement against Theresa May, producing their own alternative Brexit Bills to prevent a Westminster “power grab”.

In the most profound constitutional clash since the start of devolution, Edinburgh and Cardiff said they had been forced to activate the contingency plan to protect their devolved laws.

The move is intended to ramp up pressure on the Prime Minister to make substantial changes to the EU (Withdrawal) Bill in the House of Lords later this month.

If she refuses, and Holyrood and Cardiff Bay pass rival versions of the Bill, the dispute could end up in the UK Supreme Court, worsening an already unpredictable Brexit process.

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The Herald understands Mrs May, who is due in Aberdeen this weekend for the Scottish Tory conference, will meet Nicola Sturgeon soon after her return to Downing Street in an attempt to broker a peace between the two governments.

The development follows a year-long row between London and the devolved administrations over where devolved powers repatriated from Brussels in March 2019 should go and when.

The UK wants some key powers to go at Westminster - against the grain of the devolution settlement - pending the creation of UK-wide frameworks in agriculture and fishing to protect the UK single market.

However Edinburgh and Cardiff want all such powers devolved at the point of Brexit.

Ms Sturgeon yesterday said it was “very likely” the two sides could not agree and Holyrood would refuse legislative consent for the EU (Withdrawal) Bill, triggering a constitutional crisis.

Although Holyrood cannot veto UK Brexit law, Westminster has never legislated before without its consent on a devolved matter.

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The First Minister said her Brexit Bill was an insurance policy against such a scenario, allowing Holyrood to “provide continuity of law in devolved areas” without Westminster.

SNP Brexit Minister, Michael Russell, told MSPs he regretted bringing forward the emergency legislation, but he had been compelled to prepare for the possibility of continued deadlock with London and the erosion of the Scottish Parliament’s powers.

He said the European Union (Legal Continuity) (Scotland) Bill would mirror legislation at Westminster by transferring EU law into Scots law, to ensure continuity after Brexit, but would only cover devolved areas, leaving reserved powers to the EU (Withdrawal) Bill.

He said his preference remained to work with the UK government to amend the EU (Withdrawal) Bill, and if that was done the Holyrood Bill would be withdrawn.

Failing that, the SNP Bill was “essential preparation” for the “unfolding disaster of Brexit”.

However the proposal immediately ran into trouble, when Holyrood’s presiding officer said it was not within the parliament’s competence, as it clashed with EU law.

After “careful consideration”, Ken Macintosh said he believed the Bill wrongly assumed that Holyrood could legislate now for the exercise of powers only available to it after Brexit.

Mr Russell said ministers “respectfully disagreed” and would proceed with the Bill regardless.

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It is the first time Scottish ministers have pressed ahead with legislation in defiance of the Presiding Officer’s opinion - which is not a veto - since the parliament was set up in 1999.

The Scottish Government’s most senior law officer, the Lord Advocate James Wolffe QC, is due to tell MSPs today why he believes the Bill is within Holyrood’s competence.

The disagreement adds to the likelihood that the Bill, if passed, will be subject to a legal challenge by the UK government at the Supreme Court before Royal Assent.

The UK government could also simply ignore the continuity bill and impose its own legislation, however this would be seized upon by the SNP to promote independence.

To have any chance of success, the SNP Bill must pass before it is overtaken by the EU (Withdrawal) Bill, and so it is due to be rushed through Holyrood by March 22.

Carwyn Jones, the Labour First Minister of Wales, said it was “simply not acceptable” that UK ministers should keep control of powers in devolved areas.

He said: “We have developed our Bill to prepare for a situation where the UK government does not adequately amend its legislation to respect the devolution settlement.”

Scottish Secretary David Mundell said Brexit would bring a “significant increase in Holyrood's decision-making powers”, but insisted the UK retained control over some key powers.

He said: "We have made a considerable offer to the devolved administrations on amending the EU (Withdrawal) Bill, and look forward to further constructive talks."

The Scottish Conservatives said the Holyrood Bill was “unwelcome and unnecessary”.

MSP Adam Tomkins said: “Up until now there has been a constructive approach from both the UK and Scottish governments. A fix to make this process fit for purpose is within reach. The SNP must now reflect on whether this move will help or hinder the process.”

However Green MSP Patrick Harvie, whose party gives the SNP the numbers to pass the Bill, said it was a necessary response to the UK government treating Scotland “with contempt” through the Brexit process and failing to fix its own “dog’s breakfast of a Bill”.

Labour MSP Neil Findlay blamed the Tories for forcing Holyrood to pass its own Brexit Bill.

He said: “The Tories playing fast and loose with devolution just plays into the Nationalists’ hands, with the SNP seeking to use this situation for narrow party advantage.”

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He also criticised the SNP Government for failing to secure the PO’s endorsement for its Bill, whereas the Welsh Government’s continuity Bill had been declared legislatively competent.

LibDem MSP Tavish Scott said the failure so far by the UK and Scottish governments to reach an agreement on powers was “extremely disappointing”.

He said: “Ministers must ensure that they are committed to good governance in the process that they’re asking Parliament to endorse. We are being asked to make a huge decision about devolution itself and it will be essential that we have the fullest parliamentary scrutiny.”