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 polls for the referendum, the first in the Netherlands for more than 200 years, open at 0730 (0530GMT).

Influence

"The future of the Netherlands is in Europe," Prime Minister Jan Peter Balkenende said on Dutch television late on Tuesday.

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

Mr Balkenende assured voters the country would not lose its position and influence within the EU by accepting the constitution.

He urged them not to follow the French lead in rejecting the treaty.

Domino effect

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.

Eight national referendums are still to come.

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.

Interference

But the BBC's Geraldine Coughlan in The Hague says voters are unhappy about higher prices since the euro and discontented with the centre-right government.

Many also feel threatened by what they see as a superstate that will interfere with liberal policies such as those on gay marriage and euthanasia.

Others disagree with the swift enlargement of the EU, and oppose the possible inclusion of Turkey - also thought to be a key factor in the French referendum.

However, government and major opposition parties, making up 80% of the country's MPs, support the draft constitution in the belief that it will enhance Dutch influence in Europe.

The government says it will accept the result if the turnout is more than 30%.