THERESA May is on course for the constitutional clash of the century after the First Ministers of Scotland and Wales said they would not consent to the “naked power grab”of the Tory Brexit Repeal Bill.

In a joint statement, Nicola Sturgeon and Carwyn Jones said the Bill to take the UK out of the EU, published yesterday, was an “attack on devolution” that would weaken the legislatures in Edinburgh and Cardiff.

Scottish Secretary David Mundell said the Bill was “not a power grab but a power bonanza”.

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The Bill, if passed as is, will see no new powers automatically repatriated from the EU to Scotland after Brexit, and in some cases could even lead to less control.

The powers Brussels has over Scottish Parliament responsibilities will first be sent to Westminster, where the Tory government will decide what can and can’t be trusted to MSPs.

Because of this Holyrood will, in some cases, need to ask for permission before it can legislate on devolved issues where EU rules have previously applied, such as agriculture and fisheries.

The European Union (Withdrawal) Bill, as the Great Repeal Bill is now known, will not just repeal the European Communities Act 1972, which took Britain into Europe four decades ago, but will hand ministers sweeping so-called Henry VIII powers for two years after Brexit to correct any “deficiencies” arising from the transfer of power.

“It is one of the most significant pieces of legislation that has ever passed through Parliament and is a major milestone in the process of our withdrawal from the European Union,” Brexit Secretary David Davis said.

“By working together, in the national interest, we can ensure we have a fully functioning legal system on the day we leave the European Union.”

In their joint statement, Sturgeon and Jones said they had tried working with the Government but found that to be a “one way conversation”.

“This week began with the Prime Minister calling for a constructive and collaborative approach from those outside Whitehall to help get Brexit right.

“[The] publication of The European Union (Withdrawal) Bill is the first test as to whether the UK Government is serious about such an approach. It is a test it has failed utterly.

“We have repeatedly tried to engage with the UK Government on these matters, and have put forward constructive proposals about how we can deliver an outcome which will protect the interests of all the nations in the UK, safeguard our economies and respect devolution.

Regrettably, the Bill does not do this. Instead, it is a naked power grab, an attack on the founding principles of devolution and could destabilise our economies.”

Briefing journalists earlier in the day, Mundell said: “It is a power bonanza for the Scottish Parliament because after this Bill has been implemented, the Scottish Parliament will have more powers and responsibilities than it has today and I’m happy to be held to account for that statement once the process has been delivered.

“Needless to say there will be a process row with the Scottish Government because the Scottish Government does process row, that is their speciality.”

However, Sturgeon and Welsh Labour leader Jones joining together will make it harder for May and the Tories to dismiss the SNP’s leader’s rejection of the Bill as a push for independence. In an interview with STV Sturgeon said the fact that she and Jones had put political differences aside “underlines how unacceptable this Bill is in its current form”.

Though Holyrood can withhold its consent, it cannot block the Bill, and with No 10 insisting it must be passed, the Government could, for the first time, be forced to overrule the wishes of MSPs.