The UK is being "artificially divided" by a voting system which exaggerates political differences between the four nations, a campaign group has warned.
The Electoral Reform Society (ERS) said the general election results gave the leading parties in each nation far more seats than justified by voting share.
In Wales, Labour won 37% of the vote but got 63% of the seats.
The society said the first-past-the-post electoral system could not cope with "modern multi-party democracy".
Its report - The 2015 General Election: A Voting System in Crisis - claimed the poll in May gave the "most disproportionate result in UK history".
For the first time, four different parties were the winners in the four nations of the UK - the Conservatives in England, the SNP in Scotland, the DUP in Northern Ireland and Labour in Wales.
Steve Brooks, director of ERS Cymru, said with "continuing constitutional uncertainty" about the future of the UK, it was more than a matter of being fair to voters.
"Our electoral map is deceptive - it looks like there are whole areas of monolithic blocks of Labour red, Conservative blue, or SNP yellow in the UK," he said.
"But, as we see in Wales, this isn't actually the case. Votes for different parties aren't being reflected in seats won.
"We have seen the benefits of a fairer voting system in the assembly, giving parties a greater voice. It is time Westminster caught up with the devolved nations in this regard."