With nearly all votes counted from a referendum this week, Denmark is headed toward joining the European Union's common defence policy that it long eschewed, a new example of a country in Europe seeking closer defence links with allies after Russia's invasion of Ukraine.

The electoral commission said that with ballots fully counted in 84 of 92 Denmark's electoral districts, 66.9 per cent voted in favour of abandoning the country's 30-year opt-out from the common EU policy and 33.1 per cent against.

"An overwhelming majority of Danes have chosen to abolish the defence opt-out. I'm very, very happy about that," Prime Minister Mette Frederiksen said.

"We have sent a clear signal to [Russian President Vladimir] Putin," she added. "With the decision we have made, we show that when Putin invades a free and independent country and threatens peace and stability, we will move closer together."

On Twitter, German Foreign Minister Annalena Baerbock applauded the outcome of the Danish vote. "Every step each of us takes, makes us stronger in the face of these tectonic shifts."

Ending Denmark's opt-out would have limited practical effect for either Denmark or the EU. The referendum follows the historic bids by fellow Nordic countries Sweden and Finland to join Nato - something to be taken up at a summit next month.

For Denmark, a founding member of the 30-member defence alliance, joining the EU's defence policy would have a relatively modest impact on Europe's security architecture, particularly compared to Sweden and Finland joining Nato. But Christine Nissen, a researcher with the Danish Institute for International Studies, said both moves were "part of the same story", and would strengthen military cooperation on a continent stunned by the war in Ukraine.

Denmark's Prime Minister Mette Frederiksen, centre, speaks to people while on an election campaign, in Holbaek, Denmark. Photo / AP

The main effect of abandoning the opt-out will be that Danish officials could stay in the room when EU colleagues discuss defence topics, and Danish forces can take part in EU military operations, such as those in Africa and Bosnia and Herzegovina.

It would be the first time that one of the four Danish opt-outs from the EU's Maastricht Treaty, which laid the foundation for political and economic union, is scrapped by voters in Denmark.

"I believe people have voted yes because of the war in Ukraine. The 'yes' side has tried to misuse the war in Ukraine to make the Danes feel that it is important that we stand together," said Morten Messerschmidt, the leader of the opposition Danish People's Party and a leading opponent of removing the defence opt-out.

Political leader The Red-Green Alliance Mai Villadsen hands out flyers during their NO campaign at Noerreport Station in Copenhagen. Photo / AP

One of the founding members of Nato, Denmark has stayed on the sidelines of the EU's efforts to build a common security and defence policy in parallel with the trans-Atlantic military alliance.

For decades, Europe's been a source of contention in Denmark. In 1992, voters set back plans to turn the European construction into a union by rejecting the Maastricht treaty amid widespread opposition to a federal European government that could limit the sovereignty of individual nations.

At an EU summit in Edinburgh, Scotland, later that year, European leaders agreed on a text with tailor-made provisions allowing Danes to ratify a revised treaty with four provisions.

They allowed Danes to stay out of a joint EU citizenship, justice and home affairs, the monetary union which allowed Danes to stay out of the euro and keep the krone, and defence.

The citizenship issue, which said European citizenship would not replace national citizenship, has since become irrelevant as other members later adopted the same position.

Chairman of the Danish People's Party Morten Messerschmidt hangs a poster with writing reading in Danish "Vote no to more EU". Photo / AP

But the other provisions remain intact despite efforts by successive governments to overturn them. In a 2000 referendum, Danish voters decided to stay outside the euro and 15 years later they voted to keep the exemption on justice and home affairs.

Frederiksen, who became the first Danish prime minister to win a referendum on removing an opt-out, said she was not tempted to test other opt-outs in plebiscites.

Turnout was 66.23 per cent, according to initial official figures.

- AP