The Netherlands will hold a controversial referendum next month on the EU's new partnership with Ukraine.
But Eurosceptic activists succeeded in triggering a referendum after securing about 450,000 signatures in support of their citizens' initiative.
The European Commission President, Jean-Claude Juncker, has warned that a No vote could lead to a "continental crisis".
The UK referendum campaign is now in full swing, ahead of the British in-out vote in June on whether to stay in the EU. It has totally overshadowed the Dutch referendum.
But the geopolitical stakes are high in the EU agreement with Ukraine. The ultimate aim is to integrate the ex-Soviet republic into the EU's internal market - and Russia is hostile to it.
Supporters see it as a key step to draw Ukraine out of Russia's orbit but Eurosceptics say it goes too far, too fast.
Read more: Dutch to hold stormy referendum on EU-Ukraine deal - BBC News