The EU has hit back at London's assertion that future trade might be as easy and efficient as it is now | Oli Scarff/AFP via Getty Images EU dismisses Britain’s post-Brexit customs plan Brussels reasserted its position that the terms of the UK’s divorce must be settled before they will discuss plans for future relations between the two.

The EU on Tuesday flatly rebuffed the U.K.'s first formal proposal on a post-Brexit trading relationship, saying a new position paper released by London was an effort to leapfrog divorce proceedings.

In its response, Brussels noted that withdrawal terms must be settled first — a position they believe is strongly supported by Article 50, the brief provision in the EU treaty that sets the parameters for a nation's withdrawal from the bloc.

"We take note of the U.K.’s request for an implementing period and its preferences as regards the future relationship, but we will only address them once we have made sufficient progress on the terms of the orderly withdrawal," the EU said in a statement. "An agreement on a future relationship between the EU and the UK can only be finalized once the U.K. has become a third country."

The EU also rejected the U.K.'s attempted assertion that future trade might be as easy and efficient as it is now, as a member of the bloc.

"As Michel Barnier has said on several occasions, 'frictionless trade' is not possible outside the single market and customs union," the EU statement said, referring to its chief negotiator.

Overall, the EU said it welcomed the first in a series of papers intended to lay out the U.K.'s negotiating position, which many in Brussels have viewed as a mystery.

"We see the U.K.’s publication of a series of position papers as a positive step towards now really starting phase one of the negotiations. The clock is ticking and this will allow us to make progress," the EU said.

In its statement on Tuesday, the EU subtly chastised the U.K. for its slow pace in putting forward detailed negotiating positions. Brussels noted that London's paper on an interim customs arrangement was "a first response to the series of 9 papers which the EU published before the summer."

It then listed the topics of those papers, which began by focusing on two of the withdrawal issues that Brussels views as most critical: citizens' rights and the so-called financial settlement, by which the U.K. is expected to fulfill past and future financial commitments undertaken during its four decades as an EU member. London has indicated it will not readily agree to a specific amount that must be paid to leave the EU, though officials have repeatedly said the U.K. lives up to its obligations.

In addition to its previously published paper, the EU said it is preparing another series including one that will focus on customs issues.

And even as that point seemed to reiterate the Brussels view that the U.K. was getting ahead of itself, the EU promised to closely read the U.K. paper. It also emphasized that it would keep in mind the negotiating parameters set by the 27 remaining EU countries, as well as the European Commission.

"We will now study the U.K. position paper on customs carefully in the light of the European Council guidelines and the Council's negotiating directives," the statement said.

It noted that the next round of talks begin on August 28.