Seventy-three per cent of Britons want a vote on the EU treaty

The survey is a blow to Gordon Brown's attempts to play down the significance of the constitutional agreement.

It will also revive debate over the UK's role in Europe 72 hours before the reform treaty is signed in Lisbon.

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The ICM poll for Global Vision, a eurosceptic campaign group, shows that 73 per cent of voters still want the Prime Minister to grant a referendum on the issue.

EU leaders are due in Lisbon on Thursday to mark the signing of the treaty which critics say is a threat to British sovereignty.

An administrative blunder by Downing Street has left Mr Brown facing an embarrassing choice between snubbing Parliament or missing the ceremony.

The Prime Minister's office agreed months ago that he would make his first appearance before the Commons liaison committee on Thursday morning.

But that two-and-a-half hour question and answer session coincides with the ceremonies in Portugal.

A spokesman said efforts were being made to allow Mr Brown to fulfill both commitments.

He added: "He does want to go but it may not be possible."

Foreign Secretary David Miliband is expected to endorse the treaty on Britain's behalf.

Heads of government do not generally sign EU treaties but usually watch their foreign ministers doing so.

This procedure was followed by Tony Blair for the signing of the Nice and Amsterdam treaties and by John Major for the 1992 Maastricht Treaty.

The Tories accused Mr Brown of staging a "vanishing act" to avoid attending the Lisbon ceremony.

John Redwood, the party's competitiveness guru, said: "Mr Brown is ashamed of selling Britain down the river and has once again done his vanishing act.

"It is not good enough to leave it to the office boy when you have done something so bad for Britain.

"It is time he owned up to how much power this treaty is transferring and gave the British people a vote."

The survey will be seized on by campaigners who are gearing up for parliamentary warfare when the treaty reaches the Commons and Lords for formal ratification next year.

EU leaders have admitted it is nearly identical to the earlier constitution which was rejected by Dutch and French voters.

Mr Brown's euroscepticism did not stop him endorsing the new treaty or rejecting calls for the Government to honour Labour's 2005 manifesto commitment to put it to a referendum.

The Global Vision poll found that only 9 per cent of voters believe the treaty is different to the failed constitution.

More than half thought it was identical or broadly the same.

Only 20 per cent of voters said a referendum was not necessary.

By contrast, 47 per cent wanted a looser relationship with the EU based on trade and cooperation and 23 per cent wanted to pull out altogether.

The negative mood is echoed across Europe.

An EU-wide TNS poll in March showed 75 per cent of voters in the 27 member states want a referendum.

In the UK the figure was 83 per cent.