The first ministers of Scotland and Wales have told Boris Johnson that it would be “unconscionable” for the UK to leave the European Union without a Brexit deal.

Nicola Sturgeon and Mark Drakeford joined forces to warn the new PM – who has pledged to take the UK out of the EU “do or die” – that such an approach would have “catastrophic consequences”.

They also used their letter to Mr Johnson to argue that a “significant shift” is needed in relations between Westminster and the devolved administrations – calling for their governments in Edinburgh and Cardiff to be given “proper respect”.

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Almost immediately after Mr Johnson entered Downing Street on Wednesday, SNP leader Ms Sturgeon stepped up calls for a second independence vote, telling him it was “essential” that Scotland has “an alternative option”.

Now she and Mr Drakeford, the Labour leader in Wales, have written to him to make plain their opposition to a no-deal Brexit.

The two first ministers said they were “concerned” Mr Johnson had not ruled out leaving the EU without a deal on October 31, saying if this happened “there should be no doubt that the consequences would be catastrophic for all parts of the UK”.

They stated: “It would be unconscionable for a UK Government to contemplate a chaotic no-deal exit, and we urge you to reject this possibility clearly and unambiguously as soon as possible.

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“We are also clear that the decision on EU exit must now be put back to the people.”

With the UK facing “unprecedented constitutional challenges”, the first ministers said this was “placing great strain on the relationships between our governments”.

They said a “significant shift in the culture and approach to intergovernmental relations” was needed “to ensure that proper respect is given to devolved interests and institutions”.

A review of intergovernmental relations, which was announced by Mr Johnson’s predecessor Theresa May, must be completed, they said to help “put in place more robust machinery for working together on the basis of greater equality”.

Ms Sturgeon and Mr Drakeford also urged the new Tory leader to commit that Scotland and Wales would suffer “no financial detriment” as a result of the withdrawal process, and called on his government to draw up a new immigration policy “that fully meets the distinct needs of each part of the UK”.

The two first ministers told Mr Johnson: “Your appointment provides an important opportunity to recognise the significance of these issues and put in place actions required to reset relationships for the effective governance of the United Kingdom.

“We ask that you convene a meeting of the Heads of Government as soon as possible so that we can discuss these critical issues as a matter of urgency.

“Your agreement to these steps would be an important signal to us, to our legislatures and to our peoples of the positive way in which you intend to work with us during your tenure.”