Up to 1.8 million Scottish votes were "wasted" in June's general election because they had no impact on the result, a study claims.
The Electoral Reform Society found 66.4% of votes did not go towards electing an MP under Westminster's first-past-the-post (FPTP) system.
It said there had been a "sea of wasted votes and a surge in tactical voting".
It added it was time for a proportional system for Westminster elections, similar to the one used at Holyrood.
Under the FPTP system, Labour won 27% of the votes cast in Scotland but got just 12% of the seats, while the SNP won 37% of the votes cast but returned almost 60% of the seats, according to the ERS research.
In its report, the society argued Scotland was "shifting back to multi-party politics".
It pointed out that 21 of the 59 Scottish seats changed party in this year's general election - more in relative terms than any other region or nation in the UK.
But it warned Scotland was at risk of witnessing a "lottery election", with slim victories meaning many seats hinged on just a handful of votes.
It called for Westminster to adopt a more proportional voting system, such as Scotland's Single Transferable Vote (STV) used in local elections.
Electoral Reform Society Scotland director Willie Sullivan, said: "Electors should be able to vote for parties they agree with on the broad sweep of policy, instead of feeling the need to vote tactically based on one significant issue such as independence or Brexit because they fear 'winner takes all' dominance.
"A proportional system would allow for this, create a much broader discussion of politics ensure all votes are of equal value with citizens feeling empowered to take part."
Mr Sullivan also argued there were relatively few "stable waymarks" in the current political landscape.
He said: "Shock changes can take place very quickly and are exaggerated by the electoral system.
"Victories are precarious, and the possibility of another election in the medium term could mean all-change again soon.
"Large swings, often local in effect, show that no party can expect very long in the sun.
"Banff and Buchan where a majority of over 14,000 for the SNP turned into a majority for the Conservative party of 3,600 highlights this."
He added: "Scotland returned four of the UK's top 10 smallest majorities. Those MPs' bottoms rest on very wobbly seats."