Britain's divorce from the EU will not be settled in two years, Sir Fraser has warned
Sir Simon Fraser, the Foreign Office's most senior civil servant until 2015, said “there’s no way” Theresa May will be able to negotiate both the economic and social terms of Britain’s divorce from the European Union by 2019.
Speaking on Newsnight, the Tory claimed there are "very complex" issues that need to be resolved as well as a transitional period that will be necessary until a final agreement.
The two-year countdown to Brexit will begin on Wednesday when Mrs May formally notifies the EU that Britain is leaving the crumbling bloc.
A historic letter from the Prime Minister requesting the activation of the EU's Article 50 exit process is due to be hand delivered to EU Council President Donald Tusk in Brussels.
Sir Simon said: "I think, personally, it's certain that we won't have resolved everything in the period before the expiry of the Article 50 process.
Brexit: Results of how the UK voted Mon, March 20, 2017
Much of the North East of the United Kingdom voted to leave the European Union including Sunderland, Gateshead, Darlington, Durham, Hartlepool, Middlesbrough, Stockton, Redcar and Cleveland, North Tyneside and South Tyneside, and Northumberland
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GREAT YARMOUTH: The town of Great Yarmouth on the East Coast of England voted by 72% to leave the European Union.
"As we know, the EU side want to start with negotiating the terms of the separation. That's about money. It's about the rights of people living in other countries in Europe. It's probably also about the borders – for example in Ireland.
“And the British side, on top of that, wants to move rapidly to discuss the future relationship – both political and economic – between Britain and the EU. And that is a very complex second set of negotiations.
“There's no way, in my view, that we're going to complete all that in two years, which is why we are going to have to think about transitional mechanisms or what the British side calls 'progressive implementation' of new arrangements.
"The 58-year-old said it was possible to complete the "separation arrangements" by the 2019 deadline but claimed it will not be possible to negotiate the "future relationship" in its entirety.
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Sir Simon Fraser, former Foreign Office chief said May will not be able to complete Brexit in time
The former Foreign Office chief warned that rushing the negotiating process could lead to "unsatisfactory outcomes" such as "political ill will and turbulence".
The Prime Minister's negotiating team are said to be preparing for an emotional backlash, in fear that eurocrats will try to punish Britain leaving the union.