The post-Brexit UK-EU trade deal might take 10 years to finalise and still fail, Britain's ambassador to the EU has privately told the government.
The BBC understands Sir Ivan Rogers warned ministers that the European consensus was that a deal might not be done until the early to mid-2020s.
He also cautioned that an agreement could be rejected ultimately by other EU members' national parliaments.
PM Theresa May said she wanted Brexit to be "smooth and orderly".
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In October, Sir Ivan, who conducted David Cameron's negotiation over the UK's relationship with the EU, advised ministers that the view of the 27 other countries was that a free trade agreement could take as long as a decade.
He said that even once concluded, the deal might not survive the process of ratification, which involves every country having to approve the deal in its own parliament.
It is also understood he suggested that the expectation among European leaders was that a free trade deal, rather than continued membership of the single market, was the likely option for the UK after Brexit.
Sir Ivan's private advice contrasts with ministers publicly insisting a deal can be done in the two years allowed by the triggering of Article 50 - the formal start of the process of leaving the EU.
Downing Street said he was relaying other EU members' views, rather than his own or the British government's.
A spokesman said: "It is wrong to suggest this was advice from our ambassador to the EU. Like all ambassadors, part of his role is to report the views of others."
The government is intent on persuading us Brexit can be done smoothly, and to time.
The suggestion that the UK's most senior diplomat in Brussels has privately told the government that a final trade deal with the rest of the EU might not be done for 10 years, and might ultimately fail, may give rise to more nerves.
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