As the fourth round of Brexit talks began on Monday, Barnier said any talks on a transition could only begin after agreements were made on three issues – the Irish border, EU citizens' rights and the UK's eventual financial settlement with Brussels.

Barnier, who has already insisted on all three matters being resolved before trade deal talks, said it would also be a prerequisite for talks on a transitional arrangement.

UK Prime Minister Theresa May on Friday called for a two-year post exit transition, to avoid a "cliff edge" disruption to trade between the Britain and the remaining 27 members of the bloc.

EU leaders have said any second phase of negotiations, which would include talks on a transition period and a future trade deal, would first require "sufficient progress" to be made on divorce terms.

Barnier appeared to reiterate this stance ahead of his talks with UK Brexit representative Davis, which last until Thursday.

"For me, we cannot discuss a transition period without reaching a preliminary agreement on an orderly withdrawal. That's the mandate I have, and no other," Barnier said.

Read more: German business leaders criticize UK's Brexit tactics

The UK's chief negotiator, David Davis, said Britain would "honor commitments we have made during the period of our membership." However he maintained his stance – contrary to that of Barnier – that agreement on a financial settlement could "only be done in the context of and in accordance with a new deep and special partnership."

'Little Boris' as a gift

The back-and-forth nature of the discourse between both sides was underlined when Britain's foreign secretary, Boris Johnson, on Monday received a toy table-tennis set from his Czech counterpart.

Lubomir Zaoralek said the German-made set, coincidentally called "Little Boris," was a symbolic gift.

"I know he's good at ping-pong," Zaoralek told reporters after having lunch with Johnson, who was in Prague to discuss Britain's impending exit from the EU.

rc/kms (apd, AFP, AP, Reuters)