Tusk said it is “maybe impossible” that Denmark will be able to remain part of the EU's law enforcement agency Europol, which tackles organized crime, trafficking and terrorism.
“The European Union would of course prefer to keep Denmark as close to its core as possible. At the same time, we must also respect the outcome of national referendums as well as the existing political and legal limitations,” Tusk told reporters after meeting with Danish PM Lars Løkke Rasmussen in Copenhagen.
“So it will not be easy. Maybe impossible,” he added.
Danes in December soundly rejected a proposal backed by five EU-friendly parties that would have seen Denmark replace its current blanket opt-out of EU justice rules with the kind of model used by Britain and Ireland, which choose whether or not to participate in some areas of EU policy on a case-by-case basis.
Following the outcome of the vote, there was wide disagreement over whether Denmark would be able to remain a part of Europol when new EU legislation makes the agency supranational in May 2017.
The Danish parties that advocated a ‘no' in the referendum said that Denmark would be able to stay in Europol by striking a parallel agreement, essentially arguing that the other 27 EU countries will find it in their interest to keep Denmark a part of the agency.
The pro-EU parties that wanted Danes to vote ‘yes' in the December referendum warned that it will be such a long and complicated process that the Nordic nation risks being on the outside looking in – at least for a period of time – when Europol's new structure takes effect.
See also: Europol: What's in it for the Danes?
On Tuesday, Rasmussen told Tusk that Denmark remaining an active part of Europol is “in our common interest”. While the European Council president didn't disagree, he said that cutting a special deal with Denmark may have to take a back seat to other more pressing issues, like the continent's ongoing struggle to find a solution to the refugee crisis.
“A Danish parallel agreement is maybe not at the top of the list, but it is among our priorities,” Tusk said.
Tusk also used his appearance with the Danish PM to European Council President Donald Tusk on
Tuesday criticize the former conservative mayor of London, Boris Johnson for having saying the European Union was behaving like Adolf Hitler.
"I cannot stay silent," Tusk said. "Such absurd arguments should be completely ignored if they hadn't been formulated by one the most influential politicians of the ruling party. Boris Johnson crossed the boundaries of a rational discussion, demonstrating political amnesia," Tusk added.
Johnson made his controversial remarks concerning the UK referendum set for June 23rd during an interview with the Sunday Telegraph on May 15th.
"Napoleon, Hitler, various people tried this out, and it ends tragically. The EU is an attempt to do this by different methods," said Johnson, a populist figure known for his rhetorical flourishes and historical allusions.
Tusk said Johnson's comments were irresponsible.
"The EU may be blamed for many things, but it still remains the most effective firewall against the ever dangerous and often tragic conflicts among the nations of Europe," he said.
Rasmussen also addressed Brexit, saying “I really hope that the Brits decide to stay”.
The Danish PM also used his meeting with Tusk to praise the EU for making progress on the refugee situation.
“It took some time – and if you ask me it took too long – but Europe has started to deliver on the refugee crisis. We have dramatically reduced the number of migrants by closing the western Balkan route and signing a deal with Turkey on migration,” he said.
Tusk and Rasmussen were scheduled to travel to Greenland together later on Tuesday to discuss climate change and strengthen ties between Greenland and the EU.