Jean-Claude Juncker, chief of the European Union's executive arm, says border controls will be placed on the Irish border if Britain leaves the bloc without a deal.
Juncker told Sky News on Sunday the EU has to "make sure that the interests of the European Union and of the internal market will be preserved."
How to preserve a frictionless border between EU member Ireland and Northern Ireland, which is part of the United Kingdom, is the thorniest issue in the Brexit discussions.
Juncker noted that without a border after a no-deal Brexit, an animal entering Northern Ireland could then enter the EU via Ireland without any controls. He says "this will not happen. We have to preserve the health and the safety of our citizens."
Britain is scheduled to leave the EU on Oct. 31.