The Republic of Ireland is set to reject the Lisbon Treaty, destroying ambitions to salvage a draft European constitution.

Rejection by only one of the European Union’s 27 member states would mean that the treaty — ratified by five parliaments so far — would fall.

Ireland has been the only country to put the treaty to a referendum. According to an opinion poll published today in The Irish Times, 35 per cent of people surveyed intend to vote “No” in next Thursday’s ballot, more than double the 17 per cent figure in the newspaper’s last survey three weeks ago. Support for the treaty has fallen from 35 per cent to 30 per cent, with 28 per cent undecided and 7 per cent intending to abstain.

“It will take an unprecedented swing in the last week of the campaign for the treaty to be carried,” The Irish Times says.

Pollsters said that many voters had admitted that they did not understand the treaty, or what they were voting on.

A “No” vote by Ireland would upset plans to create an EU president and foreign minister.

The Lisbon Treaty is intended to streamline the workings of the EU now that it has expanded from 15 to 27 members, which means many more issues being decided by majority voting and the effective loss of national vetoes.

Many in the “Yes” camp fear that the referendum will be a repeat of the one held in 2001 for the Nice Treaty, when voters overwhelmingly said “No”, even though most of the country’s political parties were in favour of the treaty.

The rejection was a major headache for European leaders, but was overturned when the vote was held again a year later. This may also be a factor in the current campaign, with voters believing that they can take a “guilt-free poke” — as one potential “No” voter said this week — in the knowledge that it could be overturned later.

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