Checks at the border between Northern Ireland and the Republic of Ireland will be "unavoidable" under Theresa May's Brexit plans for the UK to leave the single market and customs union, Michel Barnier has warned.
Brussels' chief Brexit negotiator also told reporters that agreement on a transitional deal after the UK leaves the bloc in March 2019 was "not a given".
After the latest round of Brexit talks between officials in Brussels, he said three "substantial" disagreements remain with the UK over plans for a transition period.
He added: "If these disagreements persist the transition is not a given."
On the issue of the Irish border he told reporters: "Any solution must be precise, clear and unambiguous."
He added: "It is important to tell the truth. A UK decision to leave the single market and to leave the customs union would make border checks unavoidable."