Brussels (AFP) - EU President Donald Tusk warned that talks on a post-Brexit trade deal and transition period would be even more difficult than a hard-won agreement on divorce terms that was sealed on Friday.
"Let us remember that the most difficult challenge is still ahead. We all know that breaking up is hard but breaking up and building a new relation is much harder," Tusk said, reading a statement.
Issuing his draft guidelines for the talks ahead, Tusk said Britain will have to follow all EU laws, including new ones, during the two-year transition period London has requested to reassure businesses and people.
He said it must respect budgetary commitments and judicial oversight during a transition in which the remaining 27 European Union member countries continue to meet and make decisions without Britain.
Britain will also have to collect EU customs tariffs and ensure all EU checks are performed on borders with third countries, according to a copy of the nine draft guidelines obtained by AFP.
In a sign of the challenges ahead, the former Polish premier said: "We need more clarity on how the UK sees our future relations after it has left the single market and customs union.
"We are ready to start preparing a close EU-UK partnership in trade, but also in the fight against terrorism and international crime as well as security, defence and foreign policy."
Such cooperation, he said, will require the adoption of additional guidelines next year.
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Tusk said time was of the essence before Britain's scheduled withdrawal from the bloc on March 29, 2019.
"So much time has been devoted to the easiest part of the task. Now to negotiate a transition arrangement and the framework for our future relationship we have de facto less than a year," he said.
The first phase of negotiations began on June 29, about a year after Britain's shock vote to leave the bloc, and finally wrapped up when British Prime Minister Theresa May rushed to Brussels early Friday.
The European Commission, the EU executive, said it felt "sufficient progress" had been made by Britain on separation issues including the Irish border, Britain's divorce bill, and citizens' rights.
The agreement paves the way for EU leaders at a summit on December 14-15 to open the second phase of Brexit negotiations, covering trade talks and a transition period.
Tusk said he has sent the leaders the draft guidelines that they will discuss next week and urged them to mandate EU chief Brexit negotiator Michel Barnier to start exploratory talks with Britain on future ties.
Barnier told a press conference that Britain's insistence on leaving the single market and customs union left the EU with no choice but to work on a post-Brexit free trade agreement modelled on the bloc's deal with Canada.
"It's not us, it's our British friends who are giving these red lines which close certain doors," Barnier said.
He insisted on the EU's own red lines on preserving the integrity of the single market with its four freedoms on the movement of goods, capital, services and labour.
"Not everyone has yet well understood that there are points that are non-negotiable for the EU," Barnier said.
But he also predicted continued EU-British solidarity on defence and foreign policy to ensure "ensure the stability of the continent," though it will no longer be governed by EU treaties.
He said the final version of the full withdrawal agreement will have to be completed by October 2018 -- less than a year away.