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EU Brexit negotiator Michel Barnier has warned that a transition period immediately after Brexit in 2019 is "not a given".
He outlined continuing disagreements between the UK and EU over issues like freedom of movement during the period.
He also said the UK's decision to leave the EU single market and customs union meant border checks at the Irish border were "unavoidable".
The transition period is expected to span 29 March 2019 to 31 December 2020.
Mr Barnier, speaking after a week of technical discussions between civil servants on both sides, said that "substantial" disagreements remained over the transition period.
These included the rights of EU citizens who arrive in the UK after the official Brexit date to keep freedom of movement rights after the transition period ends and the right of the UK to object to EU rules during the period.
"If these disagreements persist the transition is not a given," he said.
He denied he was making a "threat" but added: "You have to bear in mind what the United Kingdom has said, I have some problems understanding the UK position."
He also hit back his UK counterpart David Davis's suggestion that the European Commission had been "discourteous" by suggesting in a draft document the UK could see some benefits - such as access to the single market - withdrawn, if disputes during the transition period could not be resolved.
He said he was "not in the least discourteous or vindictive" adding: "It's totally foreign to my state of mind."