Peers vote to block MP constituency boundary changes
Peers have voted to block a reduction in the number of MPs from 650 to 600, which the coalition had originally planned to put in place for the next general election, in 2015.
The House of Lords voted by 300 to 231 to delay until 2018 a boundary review necessary to make the change.
Labour peer Lord Hart of Chilton said this would mean the review was based on a more accurate register of voters.
But he was accused of "riding roughshod" over Lords procedures.
Lord Hart had won cross-party support for his proposed amendment to the Electoral Registration and Administration Bill, aiming to postpone the current review of the UK's parliamentary constituency boundaries until October 2018 at the earliest.
BBC political correspondent Chris Mason said: "Had there been a review of the boundaries, and a reduction in the number of MPs from 650 to 600, many analysts suggest that this could have helped the Conservatives secure an additional 20 seats at the next election."
A Downing Street spokesman said the government would try to overturn the Lords amendment when the legislation returned to the Commons towards the end of its parliamentary scrutiny.
"The PM remains of the view that we should have fewer MPs to cut the cost of politics, and more equal size constituencies so that people's votes have more equal weight," he said.
But our correspondent said the reduction in seats was now "very unlikely", with some Conservatives suggesting this could make the difference between them winning or losing the next general election.
Lib Dem leader and Deputy Prime Minister Nick Clegg announced that his party would withdraw its support for the boundary review, after the coalition abandoned reform of the House of Lords.
But senior Lords officials had advised peers that the amendment was inadmissible because it was not sufficiently relevant to the main aim of the bill, which is to change the way people register to vote.
Urging Lord Hart to drop his amendment, the new Leader of the Lords, Conservative peer Lord Hill of Oareford, said it would be "unwise" to break with precedent and ignore the ruling from the clerks of the House.
The Labour peer refused, setting the stage for what BBC parliamentary correspondent Mark D'Arcy understands to be the "first occasion since the founding of the coalition in which Conservative and Liberal Democrat ministers are whipped to vote in opposite lobbies".
The vote signifies that a majority of the House of Lords, which is a self-regulating chamber, disagreed with the clerks' ruling.
Lord Hart said the 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 peer also said it would ensure that the boundary review was based on the new system of individual voter registration, to be brought in under the bill with all-party backing.
But Tory former cabinet minister Lord Forsyth of Drumlean accused Lord Hart of "driving a coach and horses" through Lords procedures and "riding roughshod" over the advice of the clerks.
He accused the Liberal Democrats of "double crossing" Prime Minister David Cameron and making "an attempt to gerrymander our constitution for political reasons".
He said they were opposing the boundary review only because Mr Clegg was "cross" about the failure of House of Lords reform.
Senior Liberal Democrat peer Lord Rennard, 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.
"Too much will be made by the media of two coalition parties going into two different lobbies today, but in countries across Europe where coalition is much more the norm, it is much more normal and people understand that different parties vote in different ways on some issues, while agreeing on packages of measures where they can find agreement in what they both consider to be in the national interest."
He said the Conservatives had blocked Lords reform and should therefore not be surprised that they were now in this position.
But author and Conservative peer Lord Dobbs accused Mr Clegg of inventing a link between constituency boundaries and Lords reform as an excuse to "exact a little retribution" from his coalition partners.
"It's nothing less than a great political sulk," he said.
If it is to become law, the amendment will need to be approved by MPs when the bill returns to the House of Commons.
The boundary changes, approved in principle by Parliament last year, would see England, Scotland, Wales and Northern Ireland lose 31, seven, 10 and two seats respectively.