Jeremy Corbyn could be Prime Minister if the UK had ditched first-past-the-post (FPTP) and introduced the same electoral system as Scotland.
A study of the general election by the Electoral Reform Society (ERS) also found huge swathes of Labour strongholds would be held by the Conservatives if Westminster adopted a proportional representation (PR) system.
But the ERS said Westminster’s “broken” FPTP elections are more like lottery than a real choice and millions of votes make no difference whatsoever.
According to its research, 6.5 million people felt compelled to vote tactically, rather than for their first choice, and the Conservatives could have won a majority if a tiny sliver of the population - 0.0016% - had chosen differently.
The results projected under different voting systems
Under its assessment of the Alternative Vote (AV), which was rejected in a 2011 referendum, Labour would have been chief beneficiaries in 2017, gaining 24 seats while the Conservatives would have doubled their majority from 12 to 24 in 2015.
But, should the UK have adopted the system used in Scotland and Northern Ireland - the single transferrable vote (STV) - Labour would have clinched the most number of seats overall.
Millions of wasted votes
The FPTS system is exaggerating divisions and is “broken” beyond repair, the ERS said.
A jaw-dropping 22 million votes cast had no impact on the result whatsoever.
The report also highlighted the huge fluctuations in results between the 2015 and 2017 general elections.
A 43.9% increase in the SNP’s vote share in Glasgow North East recorded two years ago switched to a 9.2% hike for Labour in June.
According to its YouGov survey of 13,000 voters, Labour won 29% of votes cast in the South East but got just 10% of seats, while the Tories won 34% of the North East but returned just 9% of seats.
ERS chief executive Darren Hughes said: “For the third time in a row, Westminster’s voting system has failed to do what it says on the tin - produce a strong and stable government.
“June’s election has shown first-past-the-post is unable to cope with people’s changing voting habits - forcing citizens and parties to try and game the system.
“With an estimated 6.5 million people holding their nose at the ballot box, voters have been denied real choice and representation.
“This surge in tactical voting - double the rate of 2015 - meant voters shifted their party allegiances at unprecedented rates, with the second highest level of voter volatility since the inter-war years.
“A system designed for two parties cannot accommodate these complex electoral swings.
“In the nations and regions of the UK, elections now feel more like lottery than a real choice.
“As we’ve shown, tiny shifts in the vote result in drastically different outcomes. Having results hinge on a few hundred voters is no way to run a modern democracy.
“The vast majority of votes are going to waste, with millions still stuck in the electoral black hole of winner-takes-all.”
The ERS called for a new system to be introduced to stop votes being wasted or voters being forced into tactical decisions.
Mr Hughes said: “The real question for our politicians is this: If the two main parties can gain over 80% of the vote for the first time in decades, in a system designed for two parties, and yet both still lose - when will they show the leadership the country so desperately needs and fix our voting system?”
Caroline Lucas, co-leader of the Green Party attempted to bring forward a Bill in Parliament in 2016 that would see the UK switch to a PR system and lower the voting age to 16.
The Lib Dems and the SNP supported Lucas’ motion but the Labour position was to reject the proposal.
Corbyn has previously endorsed electoral reform but stopped short of fully endorsing a change to PR, suggesting the subject needs more investigation. Some Labour MPs rebelled at the 2016 vote, however, and wrote to the Leader of the Opposition.
They included Clive Lewis, Stella Creasy and Wes Streeting.
Jonathan Bartley, Green Party co-leader: “Our broken and out of date political system is clearly failing to deliver what the country really wants.
“True democracy isn’t a two horse race where voters feel pressured to vote tactically and real representation isn’t possible when millions of votes count for nothing.
“No one should be forced to hold their noses and vote for the least worst option rather than for who they actually believe in.
“Electoral reform is well overdue and the Green Party will always fight for a fairer, more democratic voting system that would ensure Parliament looks like the people it is supposed to represent.”