Shadow home secretary David Davis conceded Gordon Brown's Government would "probably" succeed in fending off calls for a referendum on the EU constitutional treaty.
But Mr Davis said ministers would "lose the argument" and face a backlash from the public who believed they had been promised a poll.
He told BBC News 24 Sunday programme that he did not believe claims of 120 Labour rebels prepared to defy the Prime Minister.
"I think they will probably evaporate when it comes down to it. But it doesn't mean the Government will win the argument. They may win the vote. In a way, losing the argument is worse for them.
"People will say `you promised this before. This treaty is 90% the same. What are you afraid of?'."
But he insisted: "For democracy's sake, for our national constitution's sake, this is something that absolutely has to go to a referendum."
International Development Secretary Douglas Alexander told the programme: "What we have is a reforming treaty. All of those in the past have been put to parliament for proper parliamentary scrutiny. We live in a parliamentary system here in the UK. Other countries have different systems."
Mr Davis said there was no way of knowing whether one effect of the treaty could be to secure the release of prisoners such as Soham killer Ian Huntley or moors murderer Ian Brady.
The European Charter of Fundamental Rights incorporated into the treaty says sentences should not be "disproportionate".
Mr Davis said it was not clear whether that could mean the killers' release: "The simple answer is we don't know. One of the problems of European legislation is it often expands far beyond what the Government intended."
