British politicians have fired the opening verbal salvos in what promises to be a long debate over the merits of the European Union constitution approved by the region's leaders on Friday.

British Prime Minister Tony Blair took the floor in parliament to argue that the EU constitution will not erode British sovereignty.

He says that after tough bargaining on the text of the proposed treaty, Britain will protect its right to set its own policies on taxes, defense, foreign affairs, asylum and criminal law. He said opponents who argue otherwise are peddling myths.

"Now the British people have before them the text of this treaty as agreed, and it demolishes those myths," he said. "But the myths, and the propaganda that goes with them, are not really about the constitutional treaty. They are about whether Britain should, or should not be, a leading member of the European Union."

But Mr. Blair's chief political opponent, Conservative Party leader Michael Howard, had nothing but disdain for the prime minister and his arguments.

"This constitution is bad for our democracy, bad for jobs and bad for Britain", Mr. Howard said. "The prime minister has said we must separate myth from reality. All I can say is, look who is talking, the great myth-maker himself."

The debate sets the scene for what is expected to be a long and hard-fought campaign leading up to a referendum for British voters on the EU constitution.

Parties that are opposed to, or critical of, British membership in the EU had a strong showing in this month's election for the European parliament. Afterward, officials of Mr. Blair's Labor Party said the government has a big task ahead in convincing the public to support the constitution.

Mr. Blair has not set a date for the referendum, but has stressed it does not have to be held before the end of 2006. That would be after a general election in which he plans to seek a third term.