The First Ministers of Scotland and Wales have written to Boris Johnson to request an extension to the Brexit transition period.
Nicola Sturgeon and Mark Drakeford believe more time is required to complete negotiations and support businesses through recovery from the coronavirus pandemic.
The transition period is scheduled to finish on 31 December.
The joint letter warns exiting then would be “extraordinarily reckless”.
The European Union has said it is open to extending the period, but an application to do so must be made by 1 July.
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The UK government has previously rejected any calls for an extension, arguing that it would lead to further uncertainty and extra payments to the EU.
Cabinet Office Minister Michael Gove said on Friday that he had now “formally confirmed the UK will not extend the transition period and the moment for extension has now passed”.
Mr Gove added: “On 1 January 2021 we will take back control and regain our political & economic independence”.
But Ms Sturgeon said an extension was essential to avoid needless damage to Scotland’s economy at a time when Covid-19 was hitting businesses when they are most vulnerable.
In the joint letter, Ms Sturgeon and Mr Drakeford said that without an extension to the transition period, at very best there would only be a damaging “bare bones” trade deal or even worse, a no deal outcome.
They said: “We are mindful that the Withdrawal Agreement only permits an extension of the transition period if this has been agreed before the end of June.
“At the time the Withdrawal Agreement was signed, no-one could have imagined the enormous economic dislocation which the Covid-19 pandemic has caused – in Wales, Scotland, the whole of the UK, in the EU and across the world.
“While we hope that the second half of this year will see the beginnings of a recovery, we believe that exiting the transition period at the end of the year would be extraordinarily reckless.”
They said it would pile further “very significant economic and social shock” on top of the Covid-19 crisis, hitting businesses whose reserves, in many cases, have already been exhausted, leading to more business closures and redundancies.
But they said the difference was that this shock was avoidable.
The letter said: “No-one could reproach the UK government for changing its position in the light of the wholly unforeseeable Covid-19 crisis, particularly as the EU has made it clear it is open to an extension request.
“We therefore call on you to take the final opportunity which the next few weeks provide, to ask for an extension to the transition period in order to provide a breathing space to complete the negotiations, to implement the outcome, and to give our businesses the opportunity to find their feet after the enormous disruption of recent months.”