Dublin  - Just over 3 million Irish voters were called to the polls Thursday to vote on the European Union's Lisbon treaty.

Polling began at 7 am (0600 GMT) in 43 constituencies and was to continue until 10 pm (2100 GMT).

Ireland is the only one of 27 EU member states to hold a public referendum on the treaty aimed at simplifying decision-making in the bloc.

The latest opinion polls have shown the result of the referendum balanced on a knife-edge.

All the major Irish political parties have been campaigning for a yes vote. The no campaign has argued that the treaty threatens Ireland's neutrality, influence in Europe and advantageous corporate tax regime.

An Irish no would mean the treaty, a replacement for the failed EU constitution rejected by French and Dutch voters, could not go into effect.

Irish voters have caused headaches for the EU in the past, rejecting the Nice Treaty in 2001 before approving it in a second referendum a year later.

Vote counting begins Friday at 9 am with official results expected to be announced by late afternoon. (dpa)