Some of Whitehall's most senior former mandarins are calling on Theresa May to put Brexit on hold, warning the UK is not ready for the uncertainty it will create.
Crossbench peer Lord (Gus) O'Donnell, who was head of the Civil Service under Tony Blair, Gordon Brown and David Cameron, said it would be "irresponsible" for the Government to consider taking Britain out of the EU without any clear idea of the final outcome.
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His successor in the role, Lord (Bob) Kerslake, warned Britain is heading for a "blindfold" break which will lead to years of wrangling with Brussels.
Lord Kerslake said it is clear the UK is not ready to leave the EU and he called for the two-year Article 50 withdrawal process to be extended beyond the scheduled date of Brexit on March 29.
‘A perilous journey’
Writing in the Evening Standard, Lord O'Donnell said: "We are not ready to leave. And we should not leave until and unless we know we have a clearer idea where we are going."
As things stand, it remains unclear whether the Prime Minister is leading the UK towards a Norway-style close relationship with the EU or a looser arrangement like that of low-regulation Singapore, said the crossbench peer.
"The truth is we do not know and it is, I am afraid to say, irresponsible for any government to even contemplate embarking on such a perilous journey as Brexit without giving us a clearer idea of the eventual destination," he said.
In a report published by the People's Vote campaign for a second referendum, Lord Kerslake, who led the Civil Service from 2012-14, said there is no clarity about key elements of Britain's future relationship with the EU.
‘The nightmare will not end’
Lord Kerslake. Image: PA Newswire
He said the "proper functioning of government" required a proper understanding of how issues such as the future regulation of air travel and drugs, the sharing of data and criminal records, and co-operation on tackling climate change would be dealt with in future.
"Britain is divided, directionless and hurtling towards a legal deadline with no idea where we will end up after we cross it," he said.
"A responsible government should now acknowledge our predicament: we are not ready to embark on a journey when we do not know where we will end up.
"We should not leave until and unless we know where we are going.
"If we do, the only certainty is that the resources, energy and talent of our country will be consumed by Brexit for many more years to come.
"The nightmare will not end. It will simply take on another form.
"A blindfold Brexit that offers no clarity can never provide closure.
"It means the arguments about Brexit will just go on and on."