Labour peers have narrowly lost a vote which could have delayed the government's plans for a referendum on the voting system.
They wanted to refer the legislation, which has a tight parliamentary timetable, to legislative experts.
But they lost by 224 votes to 210. The Tories accused them of "parliamentary mischief" designed to delay a bid to equalise constituency sizes.
The bill must go through quickly if a referendum is to be held on 5 May 2011.
Electoral reform was a key demand of the Liberal Democrats before they agreed to join the coalition with the Conservatives.
The Parliamentary Voting System and Constituencies Bill sets up a referendum on the voting system for general elections, which could see the first-past-the-post system replaced with the Alternative Vote, when candidates are ranked in order of preference.
The Lib Dems want to change the voting system, while most Conservatives will campaign to keep first-past-the-post.
The bill also reduces the number of MPs and redraws constituency boundaries - something Labour opposes and argues is designed to rig the electoral system.
Ministers are determined to push their plans through Parliament quickly, with a referendum on the alternative vote system pencilled in for 5 May and planned boundary changes due to be completed by 2013.
But they have only until 24 February to get the referendum legislation on the statute book in order to comply with an Electoral Commission ruling.
Former Lord Chancellor, Labour peer Lord Falconer had hoped to use an unusual parliamentary procedure which would have put the plans on hold, by arguing that it identified two parliamentary seats "for special treatment".
He argued that made the bill "hybrid" legislation - meaning it made reference to local issues as well as dealing with the UK-wide system - and wanted parliamentary clerks known as "the Examiners" to study the proposals.
Lord Falconer said he disagreed with the decision of the Clerk of Public and Private Bills who had said it was not a "prima facie" hybrid bill and quoted a 1977 bill and a 1988 Speaker's ruling to argue that it was.
He said he did not oppose treating Orkney and Shetland and the Western Isles as special cases - but he said there were others, like Anglesey and the Isle of Wight, who should be given the opportunity to be treated differently as well. The MPs for some of those areas had "complained bitterly", he said.
Lord Falconer said: "I am not flying off on my own in relation to this, I reflect the views of many people who would like to argue that places like Anglesey, or the Isle of Wight or Devon and Cornwall should have special recognition for their community position."
Sending the bill to the Examiners would allow those cases to be considered and exceptions could be made "not by what appears to be the fiat of the government without explanation".
But the Conservative Leader of the Lords, Lord Strathclyde, accused him of a "piece of parliamentary mischief".
He suggested the motion was meant to delay legislation to make changes to constituencies: "Equalising constituency sizes upsets the Labour Party. We all know that Labour have long benefited from this system, no-one talked about hybridity then."
Lord Strathclyde said it was a "political tactic designed to delay a bill concerning elections to the House of Commons". Lord Falconer accused the Tory peer in return of "political burlesque".
Peers voted by 224 to 210 not to back Lord Falconer's proposal. Among those speaking against it was the former law lord, crossbench peer Lord Lloyd.
The proposed date for the referendum has annoyed the devolved legislatures in Scotland, Wales and Northern Ireland who are due to have elections that day - and some Conservatives who say it could sway the vote.