The Conservatives have vowed to fight the government over what they say is a handover of power to Brussels.
The government will go out and argue the case, said Jack Straw
Shadow Foreign Secretary Michael Ancram says the Tories will "oppose and expose" the newly agreed EU constitution.
But Foreign Secretary Jack Straw says the document is a "good package" that will be accepted by the British people.
Although some were uneasy about the EU, few thought Britain should have an
"ineffective" marginal role, he said.
Liberal Democrat leader Charles Kennedy has given his support for the treaty, and said it was now time to make the case for it in a referendum.
"It is essential for pro-Europeans to expose the hollow myths being peddled by the Conservative leadership and UKIP," he said.
And industry leaders have also welcomed the agreement. CBI director general Digby Jones said Tony Blair had protected vital business interests.
Michael Ancram questioned how Mr Straw could be claiming a victory.
"They are defending it, not on what they have won, but on what they haven't
lost. Who do they think they're duping?"
The Conservatives had earlier urged Mr Blair to veto the whole constitution.
Mr Straw said the new constitution would "create and confirm a union of proud independent nation states" and allow national parliaments to be more influential.
Acknowledging that some parts of the EU appeared inefficient and ineffective he said: "I know there is unease about aspects of the EU."
But he said the government would argue in favour of the treaty and believed Britons would accept it on its merits.
He said Britain had kept its vetoes in all areas of key concern such as economic policy, defence and foreign affairs.
It could also opt out of majority decisions on cross-border criminal matters and social security for migrant workers.
In areas where Britain had accepted to move to majority voting it was because it was in the country's best interests, he said.
Mr Straw said the only way to gain much-demanded reforms in agricultural and fishing policies was by removing the veto factor.
"Overall it's a very good package," he said.
Eurosceptics accused the prime minister of betrayal and Tory leader Michael Howard branded his stance "a put-up job".
Former Tory Foreign Secretary Sir Malcolm Rifkind said some people were "fed up" because the constitution advanced EU powers over issues such as the charter of human rights, trade union matters and immigration and asylum.
Some Labour backbenchers are in step with the Conservatives in their opposition to the constitution.
Kate Hoey claimed Labour's commitment to public services was threatened.
"This deal is bad for working people. It is essential for us to have full
control over areas like health, education and social policy," she said.
Every country must now ratify the treaty, which will require a referendum in Britain.
European Commission vice-president Neil Kinnock said the referendum was winnable if people knew the facts.
The UK Independence Party has called for an immediate referendum, but Prime Minister Tony Blair said the treaty would first go through Parliament.