Tory plans to cut the number of MPs in Wales from 40 to 30 are effectively dead after the Lib Dems voted against their coalition partners.
MPs on Wednesday voted by 334 to 292 to delay the constituency shake-up until 2018.
The Lib Dems voted in retaliation after their plans for reform of the House of Lords were dropped last year.
Further plans to redraw Welsh Assembly constituency boundaries have also been dropped, the Welsh Office has confirmed.
A Wales Office spokeswoman said: “We have always said that moving to 30 Assembly constituencies, by reinstating the link between parliamentary and Assembly constituencies, would be dependent on Parliament approving the Boundary Commissions’ proposals for new parliamentary constituencies.
“As parliament has decided to defer the current review of parliamentary boundaries it would not be in anyone’s interest at this time to change the make-up of Assembly constituencies as proposed in the green paper.
“However, the green paper did not just focus on constituencies. Other important issues, such as the length of Assembly terms, were included and it is right that we sought people’s views on them.
“We will now consider how best to move forward with these proposals.”
Caerphilly’s Labour MP Wayne David, who is also Shadow Minister for Political and Constitutional Reform, said: “Today’s victory means that the selfish and partisan Tory changes to our parliamentary constituencies have been stopped in their tracks by peers and MPs from all sides. The Tories should try to win elections fair and square and not by moving the goalposts. It would have been an insult to democracy to reduce the number of elected MPs by 50, while 117 new unelected members of the House of Lords have been created since the last election, with more anticipated. The Government is not reducing the number of Ministers and has in fact the more Special Advisors than any other government in history.
“The cut in the number of Welsh MPs was particularly unfair. Wales was to have its MPs cut by 25%. This was the most blatant example of the Tories’ gerrymandering.
“The Tories claims that cutting the number of MPs reduces the cost of politics simply doesn’t stack up. The figures show that their 117 new peers costs an additional £18million a year, which exceeds the £13.6million that might be saved had there been 50 fewer MPs. With the prospect of even more new peers in the pipeline, the Tories simply can’t claim to be reducing the cost of politics.”
Islwyn’s Labour MP Chris Evans has also welcomed the delay.
He said: “These changes threatened the very identity of our communities and had no regard whatsoever for the natural geographical boundaries of seats in these Valleys. I am glad that Islwyn will now be maintained as an entire constituency.”