In an outspoken interview, the European Commission president said the UK would be to blame for any fallout for a chaotic exit for the bloc – as the EU “did not invent the Brexit”.
Mr Juncker said he wanted to reach an agreement with the UK but the EU had to protect the safety of its citizens.
He told Sky News’ Ridge on Sunday: “We have to make sure that the interests of the European Union and of the internal market will be preserved.
“An animal entering Northern Ireland without border control can enter without any kind of control the European Union via the southern part of the Irish island.
“This will not happen. We have to preserve the health and the safety of our citizens.”
He accused some unnamed MPs of “forgetting about the history” of the border in Ireland and fired off a warning that a no-deal could see a return to the violence of the past.
Mr Juncker said: “I don’t like a border because after the Good Friday Agreement, and this Good Friday Agreement has to be respected in all its parts, the situation in Ireland has improved.
“We should not play with this… Sometimes I have to question that some people are forgetting about the history.”