The EU's chief Brexit negotiator has urged Britain to stop playing "hide and seek" and make its mind up on a realistic EU exit policy.
Michel Barnier's remarks came in a speech in Portugal on Saturday, hours after his British counterpart David Davis accused the EU of "public posturing."
The comments follow a difficult week of talks over Britain's future relationship with the EU once it exits the block in March 2019.
This week, the UK abstained to spell out a solution for avoiding a hard border with Ireland or offer positions on other crucial issues.
Barnier added he would not be intimidated by what he called a "blame game" from the UK, and urged London to realise that it could not maintain many elements of EU membership after Brexit.
Britain must "look the reality of the EU" in the face, said Barnier, who also warned that delays in agreeing on judicial oversight risk damaging any Brexit deal.
In the text of a speech to experts in EU law, he accused British leaders of failing to understand that the unique legal structures of the EU, which is a basis for trust among member states, could not be extended to a non-member.
Both the EU and UK are hoping to make progress on a treaty before Prime Minister Theresa May meets the other 27 EU national leaders in a month. They hope to agree on a treaty by October.
Barnier said he was ready to have “political level” talks, 10 months before Britain is due to leave in March 2019, while the UK government is debating whether to drop its rejection of a customs union.
Barnier said: “If the United Kingdom would like to change its own red lines, it must tell us. The sooner the better.”