Plans to create an MP constituency straddling the Devon and Cornwall border were put on ice after peers last night voted to delay a boundary review until 2018.
A Lords amendment to the Electoral Registration and Administration Bill, calling for the review of parliamentary seats to be postponed until well after the next election, was won by 300 votes to 231 – a majority of 69.
Government plans to slash the number of MPs from 650 to 600 would have created a controversial "Devonwall" constituency, breaching the historic Tamar boundary.
Deputy Prime Minster Nick Clegg's decision to order his party to oppose the changes follows the failure of his Lords reform legislation and severely hampers the chances of the Tories winning a majority at the next General Election.
It meant, in any case, the legislation was highly unlikely to be passed without Lib Dem MPs getting behind it.
Before the vote, Lord Robin Teverson, Lib Dem peer and Cornwall councillor, who previously tabled a failed amendment to protect Cornwall's boundary, said he backed the new clause.
He said: "My amendment to the original bill to prevent a joint Cornwall and Devon constituency failed by a small margin of some 29 votes.
"We now have our second chance and I will be voting again in the Lords on Monday to make sure that this process stops in its tracks.
"Cornwall will keep its representation at Westminster with six constituencies, all within our county boundaries."
During ill-tempered exchanges, Lord Hill, the Tory Lords Leader, urged peers to drop the amendment – warning it is "unwise" to break with precedent and ignore a ruling by the clerks of the House that the amendment was "inadmissible" because it was not relevant to the Bill. But peers behind the motion ignored the advice, claiming postponing the review of the parliamentary constituency boundaries for "one electoral cycle" would 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. The vote was set to be the first time since the coalition was formed that Lib Dem ministers voted against the Government in either House.
Senior Liberal Democrat Lord Rennard, a co-sponsor of the amendment, said: "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."