The 'Yes' side in the Lisbon Treaty campaign is ahead despite gains for the 'No' side over the past fortnight.
According to a poll in today's Sunday Business Post support for a 'No' vote in Thursday's referendum has risen by six percentage points to 39%.
However the Red C Poll shows the 'Yes' side is still ahead at 42%.
The Sunday Business Post's Political Correspondent Pat Leahy said in response to the poll that the outcome of next Thursday's vote is becoming increasingly difficult to call.
"These contests go down to the wire. We saw in last year's general election how it was decided in the final days of the campaign. This will be no different," Mr Leahy said.
Sinn Fein's Mary Lou McDonald has welcomed the increase in support for a 'No' vote.
"It reflects that an increasing number of people are very unhappy with a poor deal, and I hope that those people will come out and vote and send the Government back to do a better deal," Ms Mc Donald said.
However, the Minister for Foreign Affairs Miche€l Martin has dismissed the idea that the poll is bad news for the 'Yes' campaign.
"Obviously it is tight, and therefore because it is tight we are saying to people who are positive about the country's future, who have a confidence in the country's future to come out and vote 'Yes'," Minister Martin said.