"The Danish 'No' to Europe is a massive boost for the Brexit campaign in Britain as well as Marine Le Pen and other political forces who want to see Europe revert to closed nation-states based on economic protectionism and nation-first rejection of cooperating with other EU member states and, sadly at times, open xenophobia," said Denis MacShane, Britain's former Europe minister.
But British Prime Minister David Cameron, who is trying to renegotiate Britain's relations with the EU, before holding a referendum by 2017 on whether to remain a member, may point to it as a sign other countries are unhappy with the EU as it stands.
A source familiar with Britain's talks to renegotiate ties to the EU said the result did not bode well for Britain's continued EU membership, adding fear of losing control of immigration was crucial: "The same factor, the same fear."
The Danish vote comes amid heightened security fears across Europe after 130 people were killed in Paris in attacks claimed by Islamic State militants, and as Europe struggles with a huge influx of refugees from Syria and other countries.
Danes were told by the government that certain EU laws were needed to keep the country within the cross-border police agency, Europol. But instead of seeking approval for the 22 EU acts slated for adoption, Danes were instead asked to entrust to parliament the power to decide on such opt-ins.