The Dutch prime minister appealed to the nation to back the treaty but polls predict 60% of voters will oppose it.

Many who intend to vote "No" share their European neighbour's fears about EU expansion and a loss of identity. The result is non-binding, but PM Jan Peter Balkenende has agreed to abide by it.

Voting began at 0730 local time on Wednesday. Results are expected two hours after polls close at 2100.

The mood has been generally calm, with little evidence of political activity outside polling stations, he says.

"I voted 'No'. I believe the current treaty is not democratic enough," Amsterdam voter Joris Kampmeijer, 28, told AFP news agency.

"I think it's a good thing if there's a strong Europe," "Yes" voter Jaena Padberg told AP at a community school in the city. "It's good that our rights will be secured."

Prime Minister Jan Peter Balkenende voted at his local polling station near Rotterdam. "The future of the Netherlands is in Europe," he said on Dutch television late on Tuesday.

"I believe a 'No' vote is not in the interest of the Netherlands, not of Europe."

Correspondents say that a second vote against the constitution could create a snowball effect throughout the bloc.

The treaty - which aims to streamline EU institutions following the admission of 10 new members - has to be approved by all 25 member states before it can take effect.

In France, the decisive rejection of the document forced President Jacques Chirac to remove his prime minister.

He replaced Jean-Pierre Raffarin with Dominique de Villepin in a bid to prove he had listened and responded to voters' discontent.

In the Netherlands, any rejection of the constitution is unlikely to have the same impact as the Dutch vote is non-binding.