Initially not on the agenda, EU leaders discussed Nord Stream II on Friday morning as part of a wider debate on Energy Union, the bloc's energy strategy, which proposes reducing dependence on dominant suppliers.

Tusk himself coined the phrase Energy Union in 2014, when he was Polish PM, saying it’s a way to “confront Russia’s monopolistic position” in the gas sector.

He added, on Friday: “The [European] commission has assessed that if Nord Stream II were to be constructed, it would increase Europe's dependence on one supplier and concentrate 80 percent of Russian gas imports on one route.”

“It would also lead to a dominant position of Gazprom [a Russian state firm] on the German market, by increasing its share to over 60 percent.”

He noted the commission has not yet decided whether Nord Stream II complies with EU law - the so called “third energy package,” which lays down rules on ownership structure and competitors’ rights.

Germany has defended the project.

“This is first and foremost a business proposition and we now have to obviously establish the necessary legal framework conditions,” chancellor Angela Merkel said, also on Friday after the EU summit.

Nord Stream II has caused friction between northern and southern EU states.

It comes after the commission said South Stream, a Russian gas pipeline to south-east Europe, didn’t meet “third package” criteria, costing countries such as Bulgaria, Hungary, and Italy loss of future income.

“For Italy, but also for Bulgaria and other member states, it was also important to clarify why South Stream was impossible, and Nord Stream II is possible,” said Tusk, on the summit debate.

Hungarian PM Viktor Orban noted that Nord Stream II has strategic implications.

If it’s built, Russia can bypass Ukraine, costing it billions a year in transit fees. It can also cut gas supplies to former Iron Curtain states more easily.

Nord Stream II “is related to Ukraine, and South Stream, and double standards,” Orban told press. He said it’s “even more complicated than the migrant question,” the summit’s main topic.

He said he wants to hear a “reasonable argument why South Stream is bad and North Stream is acceptable.”

Merkel briefly touched on Ukraine.

“We wish, and many others have joined me in doing so, for solutions where Ukraine is not completely excluded as a transit country but where Ukraine can also play a role as a transit country, that is the political wish that we have. So now we have to wait and see how the procedures turn out for the permissions and the licences,” she said.

The summit conclusions, which Germany signed up to, don’t name Nord Stream II.

But they say: “Any new infrastructure should entirely comply with the third energy package and other applicable EU legislation as well as with the objectives of the Energy Union.”

The commission, one month ago, already committed to “assess any such project [Nord Stream II] against the European regulatory framework.”

Tusk said the EU will treat the controversy without emotion.

“There will be no favours for dominant suppliers such as Gazprom. We are not intending to penalise anyone either, just because they have a lot of gas,” he said.