UK Government lawyers are expected to lodge an unprecedented legal challenge at the UK Supreme Court in the next 48 hours to stop the Scottish and Welsh Governments’ own Brexit bills from becoming law.

The legislation seeks to protect the devolved settlement in the event there is no agreement between London, Edinburgh and Cardiff on the UK Government's flagship EU Withdrawal Bill, which is currently going through the House of Lords.

The First Minister and her Welsh counterpart, Carwyn Jones, believe this legislation is a "naked power-grab" by Whitehall.

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Mrs May strongly denies this and insists more powers will be going to Holyrood and Cardiff Bay after Britain leaves the EU in March 2019. However, she wants a temporary hold put on some 24 powers and responsibilities so that common frameworks can be agreed to ensure the important UK-wide internal market is protected.

The Prime Minister's law officers have until Wednesday to launch a legal challenge or face the Continuity bills getting Royal Assent and becoming law.

It is thought Jeremy Wright, the Attorney General, and Lord Keen of Elie, the Advocate General, Whitehall's senior Scottish lawyer, will make the court application as early as today. A ministerial statement is expected to follow at Westminster.

Last week, David Mundell, the Scottish Secretary, said that a court challenge was "almost inevitable" but suggested it was "not a big deal" as it was just a legal process.

Yet, the UK Government's own bill has still to complete its parliamentary passage and Mrs May and Mr Mundell have until May 8, the final day of Report Stage in the Lords, to get a deal with Ms Sturgeon and her colleagues as the Withdrawal Bill after this date goes to Edinburgh and Cardiff.

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Officials from all three governments are said to have engaged in intense talks over the Easter recess to try to resolve the constitutional impasse but have so far failed.

Mr Mundell has insisted he expects the talks with the devolved administrations to go "down to the wire". SNP sources have stressed that Ms Sturgeon will not budge on what she regards as a matter of principle.