Speaking at a press conference following an EU summit, European Council President Donald Tusk suggested that "the second phase [of Brexit negotiations] will be more demanding, more challenging than the first phase."
He was echoed by German Chancellor Angela Merkel, who warned that "the most difficult phase is ahead of us," with the UK due "to tell us what they want."
European Commission President Jean-Claude Juncker, for his part, confirmed that "the real negotiations on the second phase will start in March next year."
Meanwhile, Tory Eurosceptic hardliners have reacted angrily to the EU's new series of demands related to Britain, which Brussels said should implement all new EU rules created during the transition period and stick to the bloc's customs rules.
"If the acquis, the European Court of Justice and free movement remain, we would not be in an implementation period but would still be de facto in the EU," Conservative MP Jacob Rees-Mogg told Sky News.
The EU acquis is the accumulated legislation, legal acts, and court decisions which constitute the body of European Union law.
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Rees-Mogg's fellow MP Andrew Bridgen was in turn cited by The Independent as saying that all this "doesn't sound like much of a transition to me."
On Thursday, EU leaders gave a round of applause to British Prime Minister Theresa May after she delivered a speech to outline Brexit's divorce proposals and urged Brussels to give a green light to the second stage of the UK-EU talks.
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The second phase will focus on the terms of future trade relations between Britain and the EU, security issues as well as a transition period.
Brexit talks kicked off on June 19 and are scheduled to wrap up by the end of March 2019. They were preceded by a referendum on June 23, 2016, when about 51.9 percent of British voters said "yes" to their country leaving the EU.
