As a third round of Brexit negotiations between the U.K. and the EU started on Monday in Brussels, both sides have called for greater progress to be made.
The EU’s chief negotiator, Michel Barnier, said British "ambiguity" must be removed and progress on "separation" issues made before talks can begin on the future of U.K.-EU relations.
Barnier and British negotiator David Davis appeared at a short media briefing before they started negotiations.
Davis said both sides should exhibit “flexibility and imagination”.
The key issues of separation are the nature of the post-Brexit border between the Republic of Ireland and Northern Ireland and that between Gibraltar -- a British oversees territory -- and Spain.
Other issues include the post-Brexit status of EU citizens in the U.K. and of the thousands of Britons who reside in EU member states.
"We need you to take positions on all separation issues. This is necessary to make sufficient progress,” Barnier said.
“We must start negotiating seriously... the sooner we remove the ambiguity, the sooner we will be in a position to discuss the future relationship and a transitional period,” he added.
"The EU 27 [EU members excluding the UK] and the European Parliament stand united -- they will not accept that separation issues are not addressed properly,” Barnier said.
British negotiator Davis said the week ahead is “about driving forward the technical discussions across all the issues.
"We want to lock in the points where we agree, unpick the areas where we disagree, and make further progress on a range of issues," he added.
This third round of Brexit negotiations is expected to end on Thursday.
The U.K. is due to leave the European Union by the end of March 2019 after deciding in 2016 to exit the bloc after 44 years of membership.