Gordon Brown has been challenged to "trust the people" and hold a referendum on the EU Treaty.
The Conservatives have renewed calls for a countrywide vote, insisting the draft treaty published yesterday is a treaty in all but name.
Speaking to the Policy Exchange, shadow foreign secretary William Hague said the decision whether to hold a referendum struck right at the heart of the issue of trust in politics.
As the prime minister has repeatedly stressed an ambition to restore faith in politics, Mr Hague attempted to portray a referendum as part of the "change agenda".
A fundamental change in the powers and role of the UK should require the explicit consent of the British people, Mr Hague argued.
Furthermore, all the major political parties made it a manifesto pledge to hold a referendum on an EU constitution, he continued, calling on Labour to make good its promise.
"There can be no question, then, that if this new treaty is, in fact, effectively the EU Constitution by another name, that a failure to put it to a referendum would be a fundamental breach of trust between the government and voters," he concluded.
The government has argued the draft treaty agreed at the EU summit is not a constitutional document and does not need to be put forward to a public vote.
David Miliband insisted the idea of a European-wide constitution had been "abandoned".
Speaking yesterday the foreign secretary said: "I think it is a good treaty for Europe and for Britain because it takes forward institutional reform in a sensible way and undermines the arguments of those saying that there is a super-state around the corner.
"It is evident that that is not the case."
But Mr Hague said Tony Blair's so-called 'red lines' - which were intended to preserve Britain's constitutional independence and remove the need for a referendum - have emerged as increasingly holey.
He argued the clause maintaining Britain's independent foreign policy is now a declaration and not legally binding. Furthermore the government has made "extraordinary concessions" over criminal justice and common law.
Mr Hague said: "So it is clear that this new Treaty will produce effectively the same wide and fundamental changes to the EU that the rejected Constitution intended.
"With power transferred from Britain to Brussels in spades and the EU fundamentally changed there is no question but that the Constitution by another name merits a referendum."
The Conservatives argue the government's insistence this is not a constitution is undermined by other European leaders.
Irish taoiseach Bertie Ahern said 90 per cent of the original constitution is still in place while Angela Merkel said the fundamentals of the constitution had been maintained in large part.
"Gordon Brown and his colleagues have really been out on a limb in their pretence that this new Treaty is anything other than the EU Constitution by another name," Mr Hague argued.
"The German, Spanish, Danish and Finnish Governments, the European Commission and more, all agree that what has happened is little more than a repackaging of the original."
Mr Hague was speaking after the government's European minister told MPs calls for a referendum were "absurd".