Jean-Claude Juncker, the outgoing European Commission president | John Thys/AFP via Getty Images Juncker: No-deal Brexit will lead to ‘decline’ of the UK Commission president says he believes blame lies with UK, not EU.

If there is a no-deal Brexit, it would result in the "decline" of the U.K. and a "weakening of the sources of growth" on the Continent, Jean-Claude Juncker said.

In an interview published Tuesday in France's Les Echos, the European Commission president said he "did not accept" the Brexit blame game of "pinning the eventual failure of the negotiations on the EU," adding that the "original sin" lies with the U.K. not with the EU side.

He was speaking on a day when tensions increased between the two sides, with European Council President Donald Tusk accusing Boris Johnson of playing a "stupid blame game" on Brexit and demanding to know the U.K.'s intentions.

It followed a briefing from the U.K. side of a phone call between Johnson and Angela Merkel on Tuesday morning, which all but accused the German chancellor of ending the negotiations. A Downing Street official briefed some journalists that the German leader told Johnson the U.K. cannot leave the European Union without leaving Northern Ireland in a customs union with the EU. That "means a deal is essentially impossible not just now but ever," the official said.