By Tony Barber in Brussels and John Murray Brown in Dublin

The number of Irish people planning to vote against the European Union’s reform treaty in Thursday’s referendum has unexpectedly surged, causing alarm among EU policymakers.

An Irish “no” could doom the Lisbon treaty because it must be ratified by all 27 EU member states. Ireland is the only EU member country to ratify by referendum, rather than a process of parliamentary assent.

The TNS MRBI poll in Friday’s Irish Times showed that the number of dissenters had risen from 18 per cent to 35 per cent. Support for the treaty had slipped from 35 per cent to 30 per cent. Those undecided or intending not to vote represented 35 per cent, down 12 per cent from three weeks ago.

The treaty, which superseded the more ambitious draft EU constitution rejected by French and Dutch voters in 2005, aims to make the EU function more effectively. The deepest fear of EU policymakers is that an Irish rejection would spread “contagion” to the Czech Republic, Poland and the UK – three countries where the Lisbon treaty has always been contentious – and torpedo the ratification process.

“These countries will have a pretty tough time if Ireland says ‘no’,” said one EU diplomat.

The British House of Lords has yet to give its approval to the treaty, while the Czech Senate has sent the document to the nation’s constitutional court for scrutiny. In Poland, parliament has approved the treaty but President Lech Kaczynski has yet to sign it. In each country, an Irish “no” could unleash potentially unstoppable pressure for a fresh look at the treaty, EU officials and diplomats fear.

The mood in Brussels has settled into one of quiet determination rather than panic.

“Most people still think it will be ‘yes’, so long as the turn-out’s high,” one EU diplomat said. “They also think the best thing they can do is not interfere in the campaign.”

The Irish government accused anti-treaty campaigners of creating “suspicion and confusion”.

If the treaty failed, the EU would be unable to put in place measures designed to help it cope with its incorporation since 2004 of 12 states.

These include the creation of a long-term president of the European Council, an EU diplomatic service and changes in the EU’s voting procedures.