"We need to receive specific proposals from the U.K.", Juncker told a plenary session of the Parliament in Strasbourg during a speech to MEPs in which he repeated his demand for "clarity" from the British government.
The UK had no need to be "shocked", he said, that Brussels was holding the British government to its commitments made in a joint report with the commission last December to otherwise keep Northern Ireland in lock step with European Union law, as the status quo would not last beyond Brexit day on 29 March 2019.
"The Good Friday Agreement must be preserved in all its dimensions and life for citizens on both sides of the border should be the same as it is today", he said.
"We can't do it on our own".
Barnier lauded the rules the European Union created together with Britain for 44 years to create the "social market economy" that shelters citizens and workers from excesses of deregulation.
Mr Juncker said the controversial Brussels legal text demanding full regulatory alignment between Northern Ireland and the European Union "should not come as a surprise" to the UK.
"The draft protocol on Ireland should not come as a surprise or a shock it translates last December's agreement into a legal text", Juncker said. "This is a European issue".
Mr Juncker was heckled by one MEP who shouted "it is a British issue".
"As you say it is a European issue", Juncker fired back.
Negative effects from the approach of Brexit were already apparent, the BDI pointed out, with Britain dropping from second- to fifth-most-important trade partner for Germany past year.
German Christian democrat Elmar Brok, who sits on the assembly's Brexit steering committee, said it was "damage limitation".
The European Parliament's Brexit coordinator Guy Verhofstadt said talks he held with May and her Brexit minister David Davis last week had left him confident London would "see the advantages of such an approach".
'I think the United Kingdom side have to understand that the opposite is also not possible - you can not have the rights of Norway and then the obligations of Canada'. "That's also not possible", Verhofstadt said.
But mounting differences over Britain's divorce from the European Union have led the bloc to warn repeatedly that time was running out, that the post-Brexit transition was not a given, and that London should be more precise and realistic on what sort of future ties it sought.