As 10 new members of the EU formally join the club the prime minister has stepped up his pro-European campaign.

Writing in the Times on Friday, Tony Blair said that the UK should "take pride" in its support for enlargement.

And Saturday's historic enlargement will boost the UK's position, Blair added.

"I believe, too, that the accession tomorrow will be a catalyst for change within the EU, helping to give a new push to Britain's agenda and fresh impetus to priorities which are already in the ascendancy in Europe."

The prime minister said the accession states "share the same vision for Europe's future direction".

And there was also a pledge to the new EU members that Britain "will be your allies and partners in making the new Europe work".

Blair said the UK would also gain from access to a market of 450 million consumers and the entrenchment of freedom in Europe.

"If Europe's expansion throws up problems we can and will deal with them," he wrote.

"But for every disadvantage, there will be many benefits.

"Tomorrow's expansion of the EU to the east and south is good for Europe and good for Britain. Good for our security, for our jobs and prosperity."

But as the prime minister sought to spell out the benefits of the EU, a new poll indicated the depth of public doubts about plans for a new constitution.

A YouGov survey for the Telegraph found that 51 per cent would vote "no" while just 23 per cent would say "yes".  A further 21 per cent are in the "don't know" camp.

And indicating that winning a referendum could prove difficult for the government, just 36 per cent of those polled thought that EU membership is "a good thing".

Paralysis warning

With an uphill battle to win over a sceptical public, a senior Labour figure warned the prime minister of the consequences of a "no" vote.

Speaking at the Wales TUC annual conference, European commissioner Neil Kinnock said that a constitution would "define the shape of the union, the nature of the union, over the coming decades".

"A 'no' vote means rejecting that and instructing a British government to veto the treaty," he warned.

"The resulting paralysis would be unsustainable. To break the deadlock, a second 'Are you really sure?' referendum could be held - but that, clearly, would be a sour shambles.

"More likely, because the deadlock would not be maintained, would be a different referendum on getting out of the EU or staying in, treaty and all."