The 2017 general election has been branded the “hold your nose” poll after experts estimated 6.5m people voted tactically.
The June election was found to be the second most volative in history with voters switiching parties at an “astonishing” rate, the Electoral Reform Society found.
With Labour and Conservatives securing more than 80 per cent of the vote, this year’s general election has been described as the return of two-party politics.
But the ERS insisted this was driven by tactical voting with one in five voters casting their ballot to defeat a candidate they opposed.
It also highlighted seats including York Central where other parties decided not to stand to allow their supporters to vote Labour or Conservative.
The ERS’s report, published today, argues the 2017 election showed the first past the post voting system used for Westminster elections is in urgent need of reform.
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 ERS report argues the general election results suggests “a realignment of British politics” is underway in the aftermath of the Scottish independence and EU referendums.
Issues such as globalisation and immigration are challenging traditional party lines, it said, leading to the Conservatives winning in Mansfield and Stoke-on-Trent South while Labour triumphed in Kensington and Canterbury.
The ERS highlighted how the Conservatives would have secured a Westminster majority if they had won just 533 extra votes in highly marginal seats including Keighley.
Under first past the post, votes for candidates who do not win, and those which give the winning MP a majority of more than one vote, are effectively wasted, the ERS said.
It calculated more than 1.5m votes in Yorkshire were wasted.
Mr Hughes said: “The vast majority of votes are going to waste, with millions still stuck in the electoral black hole of winner-takes-all. A diverse and shifting public having to work around a broken two-party system. The result is volatile voting, and random results in the different parts of the UK.
“There are a wide range of systems where votes are not thrown on the electoral scrapheap. “We need to move towards a means of electing our MPs where all voices are heard and where people don’t feel forced to hold their nose at the ballot box.”