The European Parliament’s Brexit chief Guy Verhofstadt is meeting David Davis and other senior UK ministers, ahead of a vote next week in Strasbourg.
Mr Verhofstadt said Theresa May must move beyond “vague aspirations” if she wants a free trade deal with the EU, after her Brexit speech on Friday.
The European Parliament is not normally directly involved with Brexit negotiations with the UK.
But MEPs will get a final veto on any trade deal.
Mr Verhofstadt is meeting Brexit Secretary David Davis in 9 Downing Street, before moving to 10 Downing Street for meetings with Cabinet Office minister David Lidington and Home Secretary Amber Rudd.
Prime Minister Theresa May is expected to “drop in” on the David Lidington meeting, Downing Street said.
Mr Verhofstadt, who last week called on Mrs May to come up with “credible proposals” for Brexit after her Mansion House speech on Friday, is to be briefed about plans for citizens’ rights, and the Northern Ireland border.
The European Parliament will finalise its latest draft proposal on the framework of the future relationship between the European Union and the UK on Wednesday, ahead of a vote next week in Strasbourg, where the Parliament meets when it is not in Brussels.
It comes as Irish Prime Minister Leo Varadkar poured cold water on a suggestion by Theresa May that the customs arrangements on the US-Canada border could provide a model for the Irish border after Brexit.
“I visited it back in August, and I saw a hard border with physical infrastructure with customs posts, people in uniforms with arms and dogs and that is definitely not a solution that is one that we can possibly entertain,” said Mr Varadkar.
Meanwhile, the DUP has stressed its opposition to the EU’s “fallback” position – which would effectively keep Northern Ireland in the single market if no other solution could be found – in talks in Brussels with EU chief negotiator Michel Barnier.
DUP leader Arlene Foster tweeted: “Just concluded a constructive meeting with Michel Barnier.
“There are sensible solutions to the border question. Greater flexibility needs to be shown by Brussels. Unacceptable for NI to be treated separately from rest of UK as set out in the draft EU legal text.”