BBC News' David Eades on the Danish referendum campaign

The latest poll by the Gallup organisation found that 47% favoured the Amsterdam Treaty changes and its plans for EU enlargement; 39% said they will vote "no" on Thursday, while 14% are still undecided.

Another poll suggested the gap was widening.

"We have to be careful all the time. We cannot take these opinion polls for sure," Prime Minister Paul Nyrup Rasmussen said.

"I am very engaged in working for a 'yes' for the benefit of my country and our European co-operation until the very last day," he said.

All members need to ratify

The Amsterdam Treaty, which was agreed a year ago, would give the EU authority over visa and asylum policies in the member countries, and paves the way for Eastern European countries to join.

The treaty must be ratified by all the 15 members before it can take effect.

Most EU countries have left it to their parliaments to ratify the treaty but the Danes are obliged to hold a referendum because it involves sovereignty being ceded to European institutions.

Six years ago, Danes rejected another key EU accord, the Maastricht Treaty, only to accept it a year later with special exemptions.

The approval for the revised treaty in a May 1993 referendum was followed by the worst riots in the country in 50 years. Police now warn that no violence will be tolerated.

The BBC correspondent in Copenhagen says many Danes believe too much power has already drifted away and the Eurosceptics have seized on the chance to use the referendum as a plebiscite on the country's membership of the EU.

Our correspondent adds, a vote against the treaty could have a devastating effect on the EU's plans for further integration and enlargement.