An overwhelming majority of Danes, almost 67 percent, voted Wednesday in favour of joining the EU's common defence policy 30 years after opting out, results showed with 100 percent of ballots counted. 

The vote comes on the heels of neighbouring Finland and Sweden's historic applications for NATO membership, as the war in Ukraine forces countries in Europe to rethink their security policy.

"Tonight Denmark has sent a very important signal. To our allies in Europe and NATO, and to (Russian President Vladimir) Putin. We're showing that when Putin invades a free country and threatens stability in Europe, we others pull together," Danish Prime Minister Mette Frederiksen told cheering supporters.

"Denmark now can partake in the European cooperation on defence and security. And for that I'm very, very happy," she said.

Denmark's defence opt-out has meant that Copenhagen, a founding member of NATO, has since 1993 not participated in EU foreign policy where defence is concerned and does not contribute troops to EU military missions.

Earlier in the day, Frederiksen had said as she cast her ballot that Denmark, a country of 5.5 million people, was "too small to stand alone in a very, very insecure world".

"There was a Europe before February 24, before the Russian invasion, and there is another Europe after," she said after the results came in.

"When there is once again war on our continent, you can't be neutral." 

A total of 66.9 percent of Denmark's 4.3 million eligible voters voted in favour of scrapping the opt-out, while 33.1 percent voted against. 

EU chiefs Ursula von der Leyen and Charles Michel welcomed the result.

Denmark's decision was a "strong message of commitment to our common security", von der Leyen tweeted, saying Denmark and the European Union would benefit.

"This decision will benefit Europe and make both the EU and the Danish people safer and stronger," Michel meanwhile wrote on Twitter.

The traditionally eurosceptic country has often said "no" to greater EU integration, most recently in 2015 when it voted against strengthening cooperation on police and security matters for fear of losing sovereignty over immigration.

- Danish opt-outs -
Denmark has been an EU member since 1973, but it put the brakes on transferring more power to Brussels in 1992 when 50.7 percent of Danes rejected the Maastricht Treaty, the EU's foundation treaty.

It needed to be ratified by all member states to enter into force. In order to persuade Danes to approve the treaty, Copenhagen negotiated a series of exemptions and Danes finally approved it the following year.

Since then, Denmark has remained outside the European single currency, the euro -- which it rejected in a 2000 referendum -- as well as the bloc's common policies on justice, home affairs and defence.

Copenhagen has exercised its defence opt-out 235 times in 29 years, according to a tally by the Europa think tank.

Frederiksen announced the referendum just two weeks after Russia's invasion of Ukraine, and after having reached an agreement with a majority of parties in Denmark's parliament, the Folketing.

At the same time, she also announced plans to increase defence spending to two percent of gross domestic product, in line with NATO membership targets, by 2033.

- 'Ukraine the major reason'-
"These kinds of votes are even more important than earlier. In times of war it's obviously important to state if you feel that you want to join this type of community or not," Molly Stensgaard, a 55-year-old scriptwriter, told AFP as she voted in Copenhagen's city hall. 

Nikolaj Jonsson, a 28-year-old sociology student, was however unhappy with the timing of the referendum, saying it had been called "in times of unrest to emphasise a 'yes'".

"I don't think it's fair to put this ballot right here, right now, because it pushes lots of people toward a yes who would normally be more sceptical toward the EU," he said.

The director of the Europa think tank, Lykke Friis, told AFP there was "no doubt that Ukraine was the major reason for calling the referendum".

Eleven of Denmark's 14 parties had urged voters to say "yes" to dropping the opt-out. 

Two far-right eurosceptic parties and a far-left party called for Danes to say "no", arguing that joint European defence would come at the expense of NATO, which has been the cornerstone of Denmark's defence since its creation in 1949.