(DUBLIN) - European Commission chief Jose Manuel Barroso warned Ireland on Thursday that the European Union has no plan B if Irish voters reject its controversial new treaty in a crunch upcoming referendum.

In remarks as he began a two-day visit, Barroso said a "no" vote in the June poll would be disastrous both for Ireland -- which he said had been a big winner from EU membership -- and the 27-nation bloc.

The warning came days after German Chancellor Angela Merkel visited Dublin and urged Irish voters to allow the EU to "continue to flourish" by voting in favour of the treaty on June 12.

But amid a show of emotions which could grow in the coming weeks, at least 5,000 farmers joined a demonstration in Dublin even as Barroso held talks with Irish Prime Minister Bertie Ahern.

"I don't want to give the impression that I am putting any kind of pressure on the Irish people," Barroso told The Irish Times, but added: "This time I can tell you there is no plan B."

"The Irish people will be sending a message to the rest of Europe and the wider world. I hope it will be... that you want a more efficient, effective and accountable Europe," he told a conference, the National Forum on Europe.

Passions are rising ahead of the Irish referendum on the Lisbon Treaty. Ireland is the only EU state to vote on it, and could in theory scupper the treaty altogether.

In 2001, Ireland sent shockwaves through the bloc when it rejected the EU's previous Nice Treaty on institutional reform and enlargement. That decision was reversed in another referendum in 2002.

The new EU treaty -- replacing the bloc's doomed constitution rejected by French and Dutch voters in 2005 -- was agreed last December amid much fanfare. It aims to prevent decision-making gridlock in the expanding bloc.

A poll published Monday showed that a vast majority of Irish voters remain undecided: 28 percent plan to vote "Yes", 12 percent "No" and 60 percent are still to make up their minds, according to the poll in the Irish Sun newspaper.

The government is keen to reassure voters on three key areas which "No" campaigners could use to oppose the EU treaty, according to observers: the issue of abortion, Ireland's long-cherished military neutrality, and tax rules.

Speaking at a conference on Europe in Dublin, Barroso said: "I can categorically state that there is nothing in the new treaty that will affect Ireland's tradition of military neutrality."

Foreign Minister Dermot Ahern told the forum the treaty would secure Irish neutrality and sovereignty over tax affairs.

"Ireland has nothing to fear and everything to gain by supporting the treaty," Ahern said.

As Barroso's visit got under way thousands of farmers rallied in central Dublin to demonstrate against current European proposals in negotiations on world trade, which they fear could threaten their welfare.

Ireland has long been a model of the benefits of EU membership, turning itself from a backwater into the so-called Celtic Tiger economy amid a huge influx of immigrants, notably from the EU newcomer states of central Europe.

But the boom times are clearly over, and some pro-EU politicians fear that the EU referendum could act as a focus for protests against looming economic woes.

Ahern announced this month he will stand down in May to fight allegations of financial irregularities. His expected successor, Finance Minister Brian Cowen, has vowed to make securing a "Yes" vote his first priority.

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