The Conservatives want a referendum on the treaty
Peers have rejected an attempt in the Lords to put into law a requirement that a referendum is held before Britain joins the euro.
The proposal by Tory peer Lord Blackwell was defeated by 60 votes.
Tory leader in the Lords, Lord Strathclyde, said there was "something very fishy" about the government's refusal to back its own policy.
Peers were debating the ratification of the EU treaty - but a vote is not expected on that at this stage.
Labour promised a referendum on Britain joining the euro when it came to power in 1997.
But in June 2003 the then Chancellor Gordon Brown ruled that the conditions for Britain's entry had not been met - effectively knocking it off the political agenda.
With the Tories seeking to keep pressure on the government over calls for a referendum on the EU treaty, Lord Blackwell tabled an amendment to prevent any "backsliding" on its euro vote commitment.
The motion was defeated by 195 votes to 135.
Lord Strathclyde said: "This astonishing vote will arouse deep suspicion in the minds of the public in view of the broken promises from Labour and Liberals over a referendum on the Constitutional Treaty.
"It would be the easiest thing in the world for the government to have backed us in writing this commitment into law. After all, they claim it is their policy. There is something very fishy indeed about their refusal to do so."
Peers also voted late on Tuesday on an amendment by UK Independence Party peer Lord Pearson calling for a referendum on Britain's continued membership of the EU.
Lib Dem peers abstained - despite their party's backing for such a referendum.
A Lib Dem spokesman said the party did not want to "give succour" to eurosceptics by voting with the UK Independence Party.
He said the Lib Dems wanted a referendum on whether Britain should remain in Europe from a "pro-European stance".
But Lord Pearson accused the party of "dishonesty".
"The Lib Dems do not know whether they are coming or going," he commented after the vote.
"They reneged on their manifesto pledge in the Commons, saying they wanted an 'In or Out' referendum and a proper debate on the EU.
"But when they are given the opportunity to vote on such a referendum, they run away from it."
The Conservatives are expected to make their final push for a referendum when the EU (Amendment) Bill reaches its report stage in the Lords in June.
But with Lib Dem peers expected to vote with the government, their chances of success are limited.
The report stage debates are expected on 4 and 9 June in the Lords, with a vote expected on 11 June - the day before Irish voters go to the polls in their referendum on the Lisbon treaty.
If the EU (Amendment) Bill clears a third reading in the Lords it will then go for Royal Assent, although the treaty could be stalled by a "no" vote in Ireland - the only EU nation holding a referendum.
It comes as the House of Commons confirmed it had instructed the Attorney General to intervene in a legal challenge by millionaire Stuart Wheeler to the government's decision not to hold a referendum on the EU treaty.
The Speaker's Office insisted the move was not an attempt to derail Mr Wheeler's challenge, which will be heard in the High Court on 9 and 10 June.
"The Speaker has no interest in the substantive issue, his sole concern is that the court proceedings respect Parliamentary privilege," a spokesman said.
It is likely to mean Mr Wheeler's legal team will not be able to use Commons documents or publications in presenting their case.
Under the 1688 Bill of Rights, Commons proceedings cannot be cited in court proceedings.
The attorney general will instruct legal counsel to act in the case, alongside Jonathan Sumption QC for the government, who represented the government during the Hutton inquiry.
Mr Wheeler insisted he was not seeking to challenge Parliament's authority, merely hold the government to account over its promise of a referendum on the EU constitution, which he argues is the same as the Lisbon treaty.
"We are not asking the Commons to do anything, all we are seeking is a declaration by the court that the government, in breaking its promise to have a referendum, is doing something unlawful," he told BBC Radio 4's Today programme.
Asked about his chances of success, he said: "If we get the declaration that we are asking the courts to make, that the refusal to grant a referendum is unlawful I don't see the government continuing taking no notice of that."