Public opinion has moved sharply against the Lisbon Treaty, with the Yes side now enjoying only a slender lead, according to the latest Sunday Business Post/Red C monthly tracking poll.

Support for the treaty has fallen from 43 per cent in February to 35 per cent today among those entitled to vote in the referendum. Those opposed to the treaty have increased from 24 per cent to 31 per cent. The number of undecided voters remains almost unchanged, at 34 per cent.

With seven weeks remaining until the expected polling date, incoming taoiseach Brian Cowen, the government and the broader Yes campaign face a difficult struggle to regain the initiative.

Today’s results are likely to cause unease in Brussels and among European governments, where a safe passage of the Irish referendum was expected. Defeat of the Lisbon Treaty would precipitate a crisis in Brussels, and would be a huge embarrassment to the Irish government.

Farmers have turned against the treaty, while Fianna Fáil voters, men and younger voters are more likely to support the proposal.

The poll has better news for Fianna Fáil when it comes to political support. The party has gained three percentage points since the last full monthly tracking poll, though the surge in support seen in an interim poll, which was taken days after Bertie Ahern announced that he was to step down, has slightly abated.

That interim poll, which showed support for the party climbing to 40 per cent, surveyed a smaller sample group than usual. However, Fianna Fáil’s standing does represent the highest level of support for the party in the main tracking polls taken over the last six months.

The state of the parties, compared to the most recent full tracking poll, is: Fianna Fáil 38 per cent (up by 3); Fine Gael 29 per cent (down by 1); Labour 10 per cent (down by 1); Greens 8 per cent (unchanged); Sinn Féin 7 per cent (down by 2); PDs 2 per cent (up by 1); Independents 6 per cent (down by 1).

The strength of Fianna Fáil’s showing confirms the boost in support enjoyed by the party following the announcement of Ahern’s impending resignation and the subsequent accession of Cowen to the Taoiseach’s office.

The poll was conducted on Monday, Tuesday and Wednesday of last week, among more than 1,000 voters across the country.