European Commission President Jean-Claude Juncker says he fears a 'No' vote in Wednesday's Dutch referendum on a trade deal with Ukraine could trigger a "great continental crisis."
The official said, "A 'No' could open the door to a great continental crisis because it destabilises Europe."
The referendum is on a treaty between the EU and Ukraine which largely deals with trade and economic issues, although it also allows visa-free travel for Ukrainians in Europe.
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It is the result of a new law in the Netherlands last year whereby a cause that gets more than 300,000 signatures triggers an 'advisory' national vote. The treaty has already been passed by the Dutch government and others, and is in force. But if more than 30 per cent of people vote in the referendum, the Dutch government has promised to respond.
Latest opinion polls on Monday suggested a big victory for the 'No side', with 47 per cent rejecting the agreement and just 36 per cent in favour.
A 'No' vote in the Dutch referendum would be widely interpreted as expressing Eurosceptic sentiment. Many predict such an outcome will boost the Brexit campaign in the UK. Britain holds a referendum on its membership of the EU on 23 June.
One EU insider said, "A 'No' vote would be taken by many back in the UK as a sign that actually this growth of Euroscepticism isn't just in the Netherlands."
Petras Austrevicius, a Lithuanian MEP who chairs Parliament's Ukraine friendship group, agreed that a 'No' vote will greatly boost the Brexit case in the UK.
He said, "Yes, I do, I regret to say. By casting their votes in this referendum on Ukraine, Dutch voters will effectively be taking part in the referendum on Brexit. Regardless of the actual motivation of the Dutch people, the arguments of the no camp will be 'copied (and pasted)' by British populists, such as Farage, for their own purposes.
"Their goal, like that of Russian President Vladimir Putin's in Moscow, is to weaken Europe. It is a cause of real concern for me that the populist parties in Europe should be so closely aligned with the Russians on such matters. I am also worried by the possibility that the Russians actively lend their financial support to these efforts as well."
He said he was also disappointed that member states like Netherlands may be putting at risk the new bilateral agreement.
"I am very much disappointed and concerned. In my view, the Netherlands, as one of the founding members of the EU, has perhaps an even greater responsibility for the EU project. Rejecting the agreement would mean rejecting the very idea of a single Europe.
"I feel that we would be condemned in the eyes of history were the agreement to be voted down for all the wrong reasons."
On Monday, British Eurosceptic MEP Nigel Farage addressed a rally of 'No' voters near Amsterdam as the campaigns for Wednesday's Ukraine referendum hot up.
Farage was a keynote speaker at the Voor Nederland (VNL) event in Volendam, north of Amsterdam, in the evening. Some 350 supporters attended.
Meanwhile, Stem Voor Nederland 'Yes' campaigners held a rally in Amsterdam's Dam Square, with Dutch defence minister Jeanine Hennis-Plasschaert, foreign minister Bert Koenders and Ukrainian foreign minister, Pavlo Klimkin.
Farage, leader of UKIP, is campaigning for a Brexit and has already played a role in the Dutch campaign. In February, Farage recorded a video saying: "I want to come and help and support…I've got everything crossed, because if you win your referendum - my goodness me - that'll help in Britain too."
He told The Sunday Times at the weekend: "If the Dutch vote decisively in a no direction, it will have an impact on the UK referendum."
Last week, some people behind the referendum caused outrage by admitting they wanted it to pressure the relationship between the Netherlands and the EU.
Arjan van Dixhoorn, Chair of Burgercomité EU, behind the referendum, admitted to NRC: "We don't really care about Ukraine."
In a late show of colours last week, Dutch Prime Minister Mark Rutte reassured voters that the Netherlands is against EU membership for Ukraine.
"We think that Ukraine should have a good relationship with Europe and Russia," he told NU.nl. "And that would not be possible if it were part of the European Union."
But former Dutch Socialist MEP Michiel van Hulten, of the 'Yes' campaign, said a 'No' vote supports Russia, adding, "The far left and far right are parroting the Russian position."
"I don't know if it's active manipulation, or if they are useful idiots."
Another 'Yes' supporter, Cees van der Sluis, said, "This is a spark that can set off problems in the whole EU," he told DutchNews.nl. "Ukraine is the victim of this anti-EU initiative, and they don't deserve it. The lack of public understanding is drastic."