EU President Jean-Claude Juncker gave an exclusive interview to Sky News' Sophy Ridge on Sunday, warning UK MPs over "forgetting" Irish history.
When questioned directly if a no-deal Brexit on 31 October would spell the return of a hard border, Juncker said "yes."
The European Commission president said he would not be the architect of a no-deal, but the bloc would be obligated to move to protect its citizens' health and safety.
"I myself am not an architect of new border stations. The British have to tell us exactly the architectural nature of this border. I don't like it, a hard border. Because after the Good Friday Agreement - and this [agreement] has to be respected in all its parts,” said Juncker.
The EU President continued, appealing to unnamed MPs:
"Sometimes I have the impression that some people are forgetting about the history. But history will be back immediately. I'm not criticising them, I have the highest respect possible for Westminster because it's the mother of all parliaments - although not being in session."
Juncker denied the EU would be responsible for the return of violence, witnessed in The Troubles when tensions ran rife between Northern Ireland and the Republic of Ireland.
"The EU is in no way responsible for any kind of consequences by Brexit - that was a British decision. A sovereign decision we are respecting but don't try to charge the EU with responsibility. The EU is not leaving the UK, the UK is leaving the EU," said Jean-Claude Juncker.
Irish border backstop
The backstop at the Irish border after Brexit has been one of the stumbling blocks of the UK-EU divorce saga. This aspect of Theresa May's deal was agreed on with Brussels but faced resistance in Britain.
Currently, people and goods can freely cross the Irish border, as both the UK and Ireland belong to the EU.
A hard border between the two Irelands would risk enflaming old animosities.
One suggested way of avoiding a hard border is to keep Northern Ireland closely aligned with the EU in certain areas while creating a border with Great Britain in the Irish Sea.
Boris Johnson's official spokesman said they were "not seeking" such an arrangement.
The UK Prime Minister is aiming for a Brexit deal but has insisted the UK will leave the EU with or without a deal "do or die" on Halloween.
Earlier this week Sky News had reported Juncker as saying he was doing "everything to get a deal" but warning that the UK leaving the EU without an agreement would entail "catastrophic consequences."
Boris Johnson, meanwhile, will try to convince key EU leaders the UK is working out a viable plan for the Irish border after Brexit, when he meets them on the margins of the United Nations general assembly in New York.
Johnson has scheduled meetings with US President Donald Trump, Germany’s Angela Merkel, French President Emmanuel Macron, Irish Taoiseach Leo Varadkar and European Council President Donald Tusk.