Brussels - The European Commission's chief negotiator Michel Barnier said on Monday he was "concerned" before heading into the third round of formal discussions regarding the terms of Britain's departure from the bloc in March 2019.
"To be honest: I am concerned; time passes quickly," Barnier said ahead of a meeting with his British counterpart David Davis.
"We must start negotiating seriously."
The EU expects Britain to use the third round of talks to clarify its positions on a number of key issues concerning its separation from the bloc, which is necessary to move on to the second phase of talks about the future relationship between the country and the EU.
The two sides will discuss citizens' rights, a financial settlement, separation issues - such as goods on market and judicial cooperation in civil and criminal matters - governance of the agreement and the issue of Ireland and Northern Ireland during this week's talks.
Flexibility and imagination
Britain however wants to begin discussions on London's post-Brexit relationship with the bloc, rather than settling separation issues first as requested by the EU.
For his part Davis reiterated the British desire to discuss "all the issues," saying that both sides needed to show "flexibility and imagination."
Barnier said that while he was ready to "intensify negotiations over the coming weeks in order to advance," the EU guidelines were clear about his mandate and the scope of the talks.
Barnier said the remaining 27 member states and the EU parliament stood united.
"They will not accept that separation issues are not addressed properly," he said.
Teams from both sides will start meeting this evening, and are set to conclude the talks on Thursday.