Tony Blair is set to sign the new EU constitution on Friday when he joins fellow European leaders in Rome.
Mr Blair decries the "myths" spread about the constitution
The prime minister has already promised a referendum before the UK ratifies the constitution so it can come into force.
Ministers say the constitution is needed for good decision-making in an EU of 25 states. But the Tories say it will be an extra burden for business.
The Rome meeting is also likely to include talks on the halted appointment of the new European Commission.
Some MEPs have objected to the appointment of Rocco Buttiglione, who recently called homosexuality a sin, as the new justice commissioner.
Incoming commission president Jose Manuel Barroso on Wednesday withdrew his line-up rather than face the possibility of the European Parliament rejecting his whole team.
The issue is expected to be discussed on the margins of Friday's meeting.
Mr Howard says the constitution could compound decline
Downing Street says it is really for Mr Barroso to find a solution.
But Mr Blair is seen as a leader who could try to persuade Italian Prime Minister Silvio Berlusconi to withdraw his nomination to the commission of Mr Buttiglione.
Friday's main business is the constitution signing ceremony.
The leaders will put their names to the document in the same room of the Campidoglio Palace used for the signing of the Treaty of Rome in 1957, which established the European Economic Community (EEC).
The document only comes into force once ratified by all the nations, which could take about two years.
The government insists it protected its "red lines" in the treaty negotiations, ensuring key issues such as defence, foreign policy, taxation and immigration will still be decided in Westminster.
It accuses its opponents of spreading "myths" about the constitution.
But Tory leader Michael Howard launched a new assault in a speech in London on Thursday, saying the constitution would compound Europe's economic competitiveness problems.
"It will be a giant ball and chain round the ankle of British business," he said.
Mr Howard said the constitution would give European judges "a pretty blank sheet of paper on which to start rewriting UK employment and trade union legislation".
He said the UK should take back powers from Brussels and let those nations who wanted to integrate further go ahead alone.
But Labour election coordinator Alan Milburn accused Mr Howard of putting British jobs, trade and prosperity at risk.
"Michael Howard is now so desperate to appeal to the UKIP vote that he has committed the Tories to an agenda of disengagement and isolationism that is completely incompatible with EU membership," he said.
'Don't be afraid'
Liberal Democrat foreign affairs spokesman Sir Menzies Campbell called the constitution a "sensible rationalisation" of existing EU treaties.
"It is in the interests of the people of the UK, and we have nothing to fear from it," he said. "No one should be deceived by Tory party scare-mongering."
Green Party MEP Caroline Lucas argued the ceremony was a "dark day" for the EU which would make the institution less democratic and commit it further to a neoliberal economic agenda.
The UK Independence Party says the constitution is another step towards a European super state. It wants Britain to withdraw from the EU completely.