Conservative leader David Cameron has accused Gordon Brown of breaking a "personal" promise to hold a referendum on the EU treaty.
David Cameron has called for a treaty referendum
"Why is he afraid to trust the people and hold that referendum?" asked Mr Cameron at prime minister's questions.
Mr Brown said a vote was not needed as Britain had secured key opt-outs.
He said Mr Cameron, who is facing pressure from MPs, was "retreating to the old agenda" on Europe, adding "the wheels are going off the Tory bicycle".
The Tory leader was attempting to regain the political initiative after backbench criticism about the direction he is taking the Conservatives and disappointing results in two by-elections.
The EU treaty was published in draft form on Monday. The Tories say it would "fundamentally change" Britain's place in the EU.
On Wednesday Mr Cameron said: "The Irish prime minister says 90% of the constitution remains in the treaty. The Spanish foreign minister says it's 98%. What figure would the prime minister put on it?"
Mr Cameron added: "He says he wants to restore trust in a government he has been part of for 10 years, he says he wants to involve people in the decisions affecting their lives, he says he wants the state to be the servant not the master.
"Yet on the key test of whether to honour the commitment he personally gave to hold the referendum he has failed.
"Why is afraid to trust the people and hold that referendum?"
The treaty, negotiated before Mr Brown took over from Tony Blair as prime minister, gives an opt-out on a human and social rights charter and keeps an independent foreign policy and tax and benefit arrangements.
Mr Brown told Mr Cameron: "You are back to the old agenda. It didn't take long after the Ealing Southall by-election for you to retreat.
"The old agenda on Europe, the old agenda on grammar schools, the old agenda on spending, the old agenda on tax cuts.
"The wheels are going off the Tory bicycle and it's just as well you have got a car following you when you go out on your rides."
After the low-key launch of the draft treaty, the EU launched three months of talks on it.