Former French President Valery Giscard d'Estaing, who chaired the 105-member European Convention that came up with the initial draft in June 2003 after two years work, welcomed the deal reached by EU leaders last Friday on a final text.
"It's a good text for Europe, a good text for Europeans," he told reporters.
He warned EU governments, though, to prepare their electorates carefully before holding plebiscites or parliamentary votes on the text, so that people do not get the wrong impression of what the constitution is meant to do.
"We should not be steamrolling over anybody," Giscard d'Estaing said.
For Britain to ratify the constitution, Blair must win a referendum. But he faces a fight from opposition politicians and a Euro-skeptic splinter group within his governing Labor Party, who claim the 465-article constitution strips Britain of its independence and confers more power to Brussels.
Britain's relationship to mainland Europe has always been a divisive issue — one of the reasons the UK has not adopted the European single currency.
Trying to get British approval for the EU constitution may prove especially difficult for Blair because conservative Britons usually dislike closer ties to Europe, while liberals increasingly dislike Blair himself for the war in Iraq.
Blair insisted Monday that Britain's sovereignty will not be eroded by the new constitution as he tried to persuade voters the historic treaty was in the country's best interests. Blair told the House of Commons that the constitution does not create a federal superstate.
"This treaty makes clear where the EU can and cannot act," said Blair, who faces a tough battle selling the constitution to his increasingly Euro-skeptic electorate.
"We pool our combined strength for our economic advantage, for influence in the world, for peace and security at home," he added. "All that is what the opponents of this treaty would put in jeopardy for the sake, not of any real British interest, but of a narrow nationalism which no British government has ever espoused, or should ever espouse, if it has the true interests of the British people at heart."
In a stormy Commons debate, Conservative Party leader Michael Howard said that the constitution was "bad for our democracy, bad for jobs and bad for Britain."
Howard, whose party opposes the constitution, said Blair did not have a mandate for signing the treaty and promised to spearhead a cross-party campaign calling for a "no" vote in a future referendum.
Blair's Labor Party won only 23 percent of the vote in the recent European Parliament elections. The Conservatives captured 27 percent of the vote, while the U.K. Independence Party, which opposes British membership of the EU, won 16 percent.
Other countries, such as Denmark, Ireland, Portugal, the Netherlands and Belgium are also holding votes on the constitution.
If approved in all EU nations, the constitution will replace the existing five treaties that govern the club, set up in 1957 to promote economic and political integration to prevent future wars.
The constitution gives more powers to the European Parliament and slims down the EU's 30-member executive commission, ensuring it can run the bloc more effectively.
It provides new voting rules to accelerate decision-making, ends national vetoes in new policy areas, including law enforcement cooperation, education and economic policy while preserving unanimity voting on foreign and defense policy, social security, taxation and culture.
The constitution also creates the post of a more permanent EU president to replace the six-month rotating system. After deadlocking at the summit, leaders cast about Monday for fresh faces to fill the bloc's top political job, with the Irish prime minister and a Spanish foreign policy expert the latest names being floated.
If approved, the new treaty would likely take effect sometime in 2007. A signing ceremony is expected to be held this autumn.
Giscard d'Estaing said the constitution was needed to "stabilize" the recently expanded EU, which grew by 10 new nations — most of them Eastern European — on May 1, and to "bring forward … political solidarity between the member states."
However Euro-skeptic parties across the EU have branded the new constitution as going too far and have already used the recent EU elections to campaign against the ratification of the text, saying it would create a European superstate.
Giscard d'Estaing said that was nonsense.
"We have never exceeded the wishes of member states," he said defending his text. "There is a gut sense among people that … we have got to be able to pull our weight with other players on the world stage."
Blair stressed the treaty preserved Britain's veto over taxation, defense and foreign policy and gave guarantees that a sweeping charter on fundamental rights would not allow European courts to challenge British labor laws, which are more restrictive than those elsewhere on the continent.
An "emergency brake" will allow Britain to challenge majority decisions on cross-border criminal matters and social security measures for migrant workers.