Polling stations have opened in Denmark for voters to decide whether to abandon their country’s 30-year-old opt-out from the European Union’s common defence policy.

The referendum is the latest example of European countries seeking closer defence links with allies in response to Russia’s invasion of Ukraine.

It follows Sweden and Finland’s historic bids to join Nato, which plans to take up their applications at the end of the month.

Some 4.2 million Danish voters are eligible to cast ballots in the referendum.

The “yes” side – in favour of getting rid of the 1992 opt-out – has been ahead in recent months.

Polls showed it with around 40% support and the “no” side with 30%.

“The world is changing and not in a good way. We need to stand together and strengthen the co-operation that strengthens our security,” Jakob Ellemann-Jensen, head of the opposition Liberal Party, said as he handed out flyers on Wednesday in a last-minute attempt to convince undecided voters to vote “yes.”

Recent polls showed that about 20% of voters remained undecided.

“Unfortunately we are looking forward to a time that will be even more unstable than what we experience now,” Danish Prime Minister Mette Frederiksen said after casting her vote. “I believe it is the right thing for Europe, I believe it is the right thing for Denmark, believe it is the right thing for our future.”

Denmark 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 are “part of the same story” and would strengthen military co-operation on a continent stunned by the war in Ukraine.

The main effect of abandoning the opt-out would be that Danish officials could stay in the room when EU colleagues discuss defence topics and Danish forces could take part in EU military operations.

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 Nato alliance.

It was one of four opt-outs that Danes insisted on before adopting the EU’s Maastricht Treaty, which laid the foundation for political and economic union.

The waiver means Denmark has not participated in the EU’s discussions on defence policy, its development and acquisition of military capabilities and its joint military operations, such as those in Africa and Bosnia.

In a 1993 referendum, Denmark also opted out of co-operation in EU justice and home affairs, the common currency and citizenship.

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

But the other provisions remain intact despite efforts by successive governments to overturn them.

Danish voters in 2000 decided to stay outside the eurozone, and 15 years later they voted to keep the exemption on justice and home affairs.