Brexit negotiator Barnier in Berlin for talks with Maas, Merkel

The European Union's lead Brexit negotiator, Michel Barnier, arrived in Berlin on Monday to meet with German Chancellor Angela Merkel and Foreign Minister Heiko Maas, as the pressure grows on London and Brussels to agree on the terms of Britain's departure.

Maas, whose country currently holds the EU's rotating presidency, said the sides are entering a "hot phase."

At the end of the year, the Brexit transition period ends and Britain will leave the EU internal market and customs union. Without an agreement in place, there is a threat of serious disruption for businesses and supply chains.

British Prime Minister Boris Johnson has previously said that a deal governing future relations with the EU must be struck by mid-October or Britain will walk away from the talks and start preparing for a "no deal" scenario.

The EU has indicated it could wait a few more weeks, but both sides still need to ratify any deal - meaning October is the only viable window.

"There are currently many unresolved issues and if we still want to achieve our goal we must make progress on these," Maas said.

A ninth round of negotiations between Britain and the EU ended last week with no breakthrough.

At the close of those talks, Barnier vowed "to maintain a calm and respectful attitude," while reporting "persistent serious divergences on matters of major importance."

The talks - intended to clinch a massive free trade deal and iron out future cooperation - came after the commission launched legal action against Britain due to its controversial Internal Market Bill.