Britain’s voting system is “splitting the union” and creating “artificial divides” between nations because it is incapable of delivering a fair result in an era of multi-party politics, according to a study of the link between votes cast and seats won at the general election.

A report by the Electoral Reform Society (ERS), which campaigns for a more proportional system, concludes that the first-past-the-post method delivered a share-out of seats at Westminster on 8 May that not only exaggerated political division in the UK but left millions feeling that their votes counted for nothing.

The report, The 2015 General Election: A Voting System In Crisis, argues that, while the union narrowly survived last year’s referendum on Scottish independence, it is under strain because the winner-takes-all system creates “artificial cleavages” which entrench division and run counter to the goals of national and regional unity.

Ahead of the report’s publication tomorrow, Katie Ghose, chief executive of the ERS, told the Observer that retention of the current system for future elections would make a mockery of David Cameron’s claim to want to build a one-nation Britain.

She said: “7 May was the most disproportionate election in British history – and it’s about time we had a fairer system for electing our MPs. First-past-the-post is artificially dividing the UK – giving the SNP nearly all Scottish seats on half the vote, while excluding Labour from the south of England and over-representing them in Wales.”

On 7 May the SNP won 95% of Scottish seats (56 out of 59) having secured just half of the vote, leaving unionist sentiment north of the border “almost completely unrepresented”, the report says.

“[In Scotland] Labour, the Conservatives, and Liberal Democrats got just one representative each for the other 50%. For 24.3% of the vote Labour returned just one Scottish MP - the same as the Liberal Democrats on 7.5%.”

Other distortions, the ERS says, were that Labour won just 8 seats in the south west and south east of England (outside London) despite gaining nearly a fifth of the share of the vote in those regions. Equally, nonsectarian parties were “locked out” in Northern Ireland.

First past the post was created to deliver a clear result in an era when Labour and the Conservatives dominated British politics, says ERS. But with their vote shares declining steadily and smaller parties increasing support it has shown itself incapable of reflecting the wishes of those who reject the “big two”. The result is a dangerous dual effect of entrenching unfairness and stoking political alienation.

Support for Ukip surged at the election, taking its vote share to 12.6%. “Yet this remarkable result delivered Ukip just one Member of Parliament,” the study says. “The Conservative party received three times as many votes, but 331 times as many MPs.” Nigel Farage has re-iterated his backing for voting reform in response to this inbalance between his party’s votes and seats. In the report the ERS calculates that, had the election been conducted under a proportional single transferable vote system, Ukip would have won 54 seats, the Lib Dems 26 instead of 8, and the Greens three instead of one.

The report adds: “The system cannot cope with the choices voters are making in this multi-party era. People are choosing to vote for a wider range of parties and our electoral system should be able to reflect that in the composition of Parliament.”

Almost three quarters of votes cast had no numerical effect on the outcome, it says. “Because FPTP is a winner takes all system, votes for the losing candidates are not taken into account in any way — something which, when replicated nationally leads to millions of unrepresented voters. It produces safe seats where large majorities mean even those voting for the winner have little effect on the actual outcome. At this election, nearly three quarters of votes were wasted in this way.”

Half of all votes (50%) went to losing candidates, representing 15 million people who did see their choice reflected in the outcome.

Ghose added: “That nearly three quarters of votes were wasted in this election shows that we have a democratic crisis on our hands, with most people’s votes not counting. We have an archaic and divisive voting system that leaves millions disenfranchised and forces millions more to feel that they have to vote for a ‘lesser evil’ – instead of who they really support. The Greens and Ukip won 5m votes and just two seats between them. This is simply unsustainable – and can only end badly.”

The ERS insists that the general election should have been the final nail in the coffin for FPTP, but the Conservative party has expressed no desire to change the system. A referendum in the last parliament, on introducing the Alternative Vote (AV) system failed to win enough public backing.

• This article was amended on 1 June 2015 to correct references to the Electorial Reform Society as the ERF, rather than ERS.

