The government has suffered a significant defeat in the House of Lords over plans to equalise parliamentary constituency boundaries before the next general election in 2015.
Peers backed an amendment by Labour peer Lord Hart of Chilton to defer the boundary review until 2018, by 300 votes to 231 - a majority of 69.
The defeat, which is likely to cause further tensions within the coalition, came during committee stage of the Electoral Registration and Administration Bill on 14 January 2013.
It is the first time since the coalition was formed in 2010 that Liberal Democrat ministers as well as backbenches have voted against the government in either House.
Deputy Prime Minister Nick Clegg's decision to order his party to oppose the changes follows the failure of his Lords reform legislation.
Lord Hart's amendment, which delays a boundary review and a cut in MP numbers from 650 to 600 until 2018 at the earliest, was ruled "inadmissible" by the clerks - and the new Lords Leader, Lord Hill of Oareford, asked him to withdraw it as it was "not relevant" to the bill.
But Lord Hart refused, insisting there was a genuine difference of opinion over its admissibility, which was for the House to settle.
He told peers his amendment would ensure the 2015 general election was contested on the basis of current boundaries and "provide a window of time to address the current deficiencies in the electoral register".
The purpose, he said, was to allow time for the new system of individual voter registration, brought in under the bill, to bed down and test how far the register was accurate.
'A great political sulk'
Tory former cabinet minister Lord Forsyth of Drumlean accused Lord Hart of "driving a coach and horses" through the procedures of the House.
Senior Liberal Democrat Lord Rennard, a co-sponsor of Lord Hart's amendment, said his party would vote for the delay.
The former chief executive of the Lib Dems said: "Many in my party take the view that the reduction in the number of MPs proposed in the boundary review should not take place without reform that would strengthen the legitimacy of this House."
Tory Lord Dobbs hit out at the Liberal Democrats and said it was an "invention" that there was a link between constituency boundaries and House of Lords reform.
"The truth is that this is solely, sadly and cynically because the deputy prime minister didn't get his way on House of Lords reform. Now he wants to exact a little retribution. It's nothing less than a great political sulk."
Opposition spokesman and former Lord Chancellor, Lord Falconer of Thoroton insisted Lord Hart's amendment was relevant and admissible, adding that to fundamentally change the system of registration was bound to have a very significant effect on the boundaries that were to be fixed.
Home Office minister Lord Taylor of Holbeach branded the attempt to amend the bill "disingenuous" and said some would say it was "conceived in mischief".
Lord Taylor said peers should be very wary of "denying the will" of Parliament in previously approved legislation on the boundary review.
To view part two of the debate click