There will have to be border checks in Ireland if there is a no-deal Brexit in order to preserve the interests of the European Union, Jean-Claude Juncker has reiterated.
The European Commission president insisted that Brussels was "in no way responsible" for the consequences of a no-deal Brexit, saying the blame would lay squarely with the UK.
But he said "we can have a deal", and Boris Johnson's proposals for dealing with the problems Brexit will create at the border with Ireland were the basis for progress.
The British Prime Minister has demanded that the backstop - a contingency plan to prevent a hard border by keeping the UK aligned with many of Brussels' rules - should be scrapped.
Mr Juncker told Sky News' Sophy Ridge on Sunday: "We have to make sure that there will be no hard and physical border between the two parts of the Irish island and things have to be done on a level playing field.
"If these three objectives are met by the alternative arrangements, then we don't need the backstop.
"The backstop was never an instrument having been put in place for whatever will happen. No, it was put in place in order to preserve the rights of the internal market and of the island of Ireland."
But in the event of a no-deal Brexit, Mr Juncker said there would have to be checks.
He added: "I'm not an architect of new border stations. The British have to tell us exactly the architectural nature of this border.
"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."
He said: "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 said the EU "is in no way responsible for any kind of consequences entailed by the Brexit" as "that's a British decision".
He suggested that some UK MPs are "forgetting about the history" of Northern Ireland and Ireland.
Mr Juncker's meeting with Mr Johnson in Luxembourg on 16 September came just hours before the country's prime minister Xavier Bettel criticised the Tory leader and left an empty podium for him at a press conference.
The European Commission president said: "I don't know if this is helpful. I rather consider that this was not very helpful but it's his decision."
Mr Juncker also criticised former UK prime minister David Cameron over the build-up to the 2016 Brexit referendum, claiming he failed to explain the agreement reached with Brussels.
"We agreed the deal back in 2016, I think, between David and ourselves but this deal was never explained to the British public. Never."