Reuters / Kirsty Wigglesworth
Prime Minister Theresa May.
Sir Ivan Rogers, who represented David Cameron during negotiations to revise Britain's membership of the EU prior the referendum, told ministers that the 27 other member states believe an agreement probably won't be reached until after 2020, the BBC reports.
He also warned the government that a proposed UK-EU trade deal could be rejected by the national parliaments of the other 27 states at the eleventh hour, as all EU members must approve a trade deal before it is finally signed off by Brussels.
The difficulty of concluding rapid talks was highlighted earlier this year when the EU-Canada free trade deal (CETA) nearly collapsed after seven years of talks after being vetoed by the Belgian region of Wallonia. The regional government eventually agreed to support the deal, but only after all other parties agreed to a series of significant revisions.
The ambassador's warning comes as May today urged other EU leaders to make Britain's exit as "smooth and orderly" as possible. "We want that to be as smooth and orderly a process as possible. It is not just in our interests, it is in the interests of Europe as well," the prime minister said as she arrived for a summit in Brussels.
"We are going to invoke Article 50, trigger the negotiations by the end of March next year and it is right that the other leaders prepare for those negotiations as we have been preparing," she added.
Unless Britain opts to retain its single market membership, which at the moment appears unlikely, it'll have to agree a whole new free trade arrangement with the EU. This process will challenge both the government and Whitehall to forge Britain's place in the world outside the EU while handling the day-to-day job of domestic governance.
May and her ministerial team have vowed to make the process as speedy as possible, but it's evident that the task facing ministers and Whitehall could easily go beyond the next general election, which is set for 2020.
Last month, the Institute for Government warned that the sheer workload of delivering Brexit could present an "existential threat" to the civil service, which has been cut down in size and resources over recent years.
Sir Ivan's claims have been shot down by pro-Leave politicians.
Tory MP Dominic Raab told BBC Radio 4's Today programme that the ambassador was "the diplomat who persuaded David Cameron to dilute his ambitions for the renegotiation which was one reason why the referendum was lost, so he's been scarred by his own pessimistic advice in the past."
However former deputy prime minister Nick Clegg, who Business Insider interviewed this week, told the BBC that Sir Ivan is an expert who ought to be listened to. "I worked with Ivan Rogers in Brussels on trade deals 20 years ago. He knows what he is talking about - the government should listen," the former Lib Dem leader said.
The ambassador's comments will likely fuel calls for the government to pursue a transitional deal, to cover the period of time between Article 50 expiring and a permanent UK-EU deal coming into effect. Speaking to the Commons Select Committee for Exiting the EU yesterday, David Davis said the government would try to agree a transitional arrangement "if necessary" but said the public wants Brexit to be completed as soon as possible.
The Supreme Court will rule next month whether May is legally required to secure parliamentary approval before triggering Article 50 of the Lisbon Treaty. The prime minister has agreed to disclose at least some of her negotiating strategy prior to talks getting underway, but how much she will reveal to MPs is yet to be seen.