The Conservatives called for a referendum on the treaty
The House of Lords has voted against holding a referendum on the EU Treaty - a day before the Irish Republic's national ballot on the agreement.
Peers dismissed a Conservative proposal by a margin of 62 votes.
The Tories argued the treaty was largely similar to the discarded EU Constitution - on which a referendum was promised by all major parties.
But the government and Lib Dems said it was not needed as the treaty would not alter the UK's own constitution.
The Lords vote - by 280 to 218 - clears the way for the bill ratifying the EU treaty to become law next week.
However, the treaty - which paves the way for greater European integration, an EU presidency and the abolition of a host of national vetoes - will effectively be killed off if Ireland votes "no" on Thursday, as it must be passed by all 27 member states.
The Lords vote came after judgement was reserved in a legal challenge brought by millionaire Tory donor Stuart Wheeler, who claims the government promised a referendum and is now obliged to hold one.
Ministers insist the pledge to hold a referendum - made by all three main parties at the last general election - related only to the EU Constitution, which was abandoned in 2005.
They argue that the current treaty is significantly different.
But, during the debate, Conservative Lord Howell said: "What has happened is that the treaty drafters and this government have achieved an illusion by using a methodological device which is used to say the constitution position is abandoned."
He added: "The public is being hoodwinked and the public knows that as well."
Labour's Lord Anderson replied that there was a "sufficient change" from the constitution to the treaty.
Nine EU members had been proposing to hold referendums on the former and now Ireland was holding one on the latter.
He added: "The democratic case for a referendum is very weak indeed."
Matters "wholly unrelated" to the substance of the treaty could be used in campaigns and influence voters, Lord Anderson said.
Lib Dem leader Nick Clegg told his peers to oppose a referendum. When MPs voted in March, Lib Dems were told to abstain.
In a letter to Mr Clegg earlier, shadow foreign secretary William Hague accused the Lib Dems of being determined to deprive the British people of a say and push the EU Treaty through "at all costs".
Addressing his fellow peers, Lib Dem Lord Wallace of Saltaire said he had been "struck by the relative modesty of the changes" outlined in the treaty, making a referendum unnecessary.
In March, MPs voted by 346 votes to 206 to approve the EU (Amendment) Bill.
The bill, if passed, will ratify the Lisbon Treaty, which was drawn up to replace the EU constitution after that was rejected by French and Dutch voters in 2005.