MPs backed a House of Lords amendment to tear up the constituency boundaries plan until 2018 – three years after the next general election.
It means Cornwall's historic border will be preserved for the time being, and makes it far more difficult for the Conservatives to win the next election outright.
But the delay prompted a huge row between Liberal Democrat and Conservative MPs in Cornwall – with the Tories accused of putting "their party's interest before that of Cornwall" and the Lib Dems condemned for being "totally inconsistent" after backing the legislation in a crucial vote previously. North Cornwall Lib Dem MP Dan Rogerson said: "There is huge opposition to this artificial boundary which takes no account of the historical, cultural and community position of Cornwall. Local people will remember that the Tories failed to support the cause when the chips were down."
All six Cornwall MPs – Conservative and Lib Dem – had defied the Government in 2010 by voting for a legislative amendment that would have protected the Duchy from encroachment, but the motion failed.
George Eustice, Conservative MP for Camborne and Redruth, said: "The Liberal Democrat position on 'Devonwall' has been totally inconsistent.
"We all voted to attach an amendment to preserve Cornwall but the Lib Dems voted for the Bill at the third reading (of the Parliamentary Voting System and Constituencies Bill).
"That was the point – to oppose it before the Boundary Commission spent millions on deciding new boundaries."
In the deepest split yet between the coalition parties, Lib Dems combined with Labour and smaller parties to delay the implementation of the boundary review – thought to be worth about 20 extra seats in the Commons to the Tories.
The plan was to slash the number of MPs from 650 to 600 and make all parliamentary seats roughly the same size.
For example, Torbay has around 10,000 more constituents than neighbouring Totnes in Devon, leading to claims by Tory MP Sarah Wollaston that those voters are "disenfranchised".
The move would have cost Devon and Cornwall one Member, and left the "Devonwall" MP representing a constituency that included Bude in North Cornwall and Bideford in West Devon. Nick Clegg's Liberal Democrats initially supported the changes as part of a package of constitutional reforms, but announced last summer that his party would try to delay the review in revenge for the Tories killing off plans to reform the House of Lords.
Last night, the Conservatives were defeated by a margin of 292 to 334. All 57 Lib Dem MPs voted in favour of the amendment. Of the four rebel Conservatives, none were from Cornwall.
The reform touched on a nerve across the region. Scores took part in an anti-"Devonwall" demonstration on the Cornwall banks of the Tamar, and Prime Minister David Cameron was criticised for a flippant attitude towards the Cornish identity when he quipped on television: "It's the Tamar, not the Amazon, for heaven's sake."
Of the 18 MPs that represent Devon and Cornwall in parliament, only four would have escaped with no changes to their patch – though none were as significant as the cross-Tamar constituency.
Ben Bradshaw, Labour MP for Exeter, said: "Although Exeter wasn't affected I am pleased this outrageous piece of gerrymandering has been defeated. It was always a blatant attempt by the Tories to skew the system in their favour, creating ridiculously artificial seats like the one crossing the Devon-Cornwall border. The process of reviewing boundaries should now be given back to the independent Boundary Commission where it belongs, safe from interference by political parties."
