Brexit: Checks at border between NI and Republic of Ireland 'unavoidable' - Barnier
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.
Advertisement
Hide AdAdvertisement
Hide AdHe 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."