The government has suffered a defeat in the Lords - by a majority of just one vote - on whether there should be a threshold on turnout at the forthcoming referendum on changing the system used to elect MPs.
As peers began committee stage debate on the Parliamentary Voting System and Constituencies Bill on 7 February 2011, peers backed by 219 votes to 218 an amendment tabled by former Labour minister Lord Rooker to make the referendum binding only if turnout was 40% or more.
It is the government's third defeat in the Lords on the bill, which MPs have already passed, and which peers spent a marathon 17 days considering at committee stage.
Under the coalition's plans, voters will decide on 5 May whether to move to the alternative vote (AV) system or retain first past the post.
Labour justice spokesman Lord Falconer of Thoroton said the amendment would give ministers discretion to proceed with AV if a majority voted in favour of the change, but turnout was below 40%.
The bill combines the Lib Dem policy of an AV referendum with the Tory policy - which Labour opposes - of reducing the number of MPs from 650 to 600 and making constituency sizes more equal.
Ministers will have to decide whether to ask MPs to overturn the defeats when the bill returns to the Commons next week.
Lord Rooker, opening the debate, said the amendment "does not prevent the referendum taking place. It does not have a threshold which stops the change in the bill, as proposed, taking place".
"It simply allows the compulsory change in the bill to be activated only if the turnout is 40% plus. If it is less than 40%, it still allows the change - but requires the decision of a minister to do so."
For the government, Advocate General Lord Wallace of Tankerness said: "We believe that the certainty of the will of the people should be given effect without further complex procedures, without further parliamentary debate or political wrangling."
Lord Wallace, a Liberal Democrat, added: "I believe the bill as it stands offers simplicity, above all it offers certainty.
"Every vote will count, not be distorted by any artificial barrier or threshold."
Watch the second part of the debate