The UK's political system is "at breaking point" as support for the Tories and Labour plunges to record lows, electoral reform campaigners have said.
Recent national polling puts the combined vote share of the two biggest parties well below 50 per cent after voters expressed their frustration with the stalled Brexit process.
Labour and the Conservatives lost votes across the country at last month's European elections which saw the emergence of Nigel Farage's Brexit party and a revival of the Lib Dems.
Three consecutive polls published last week have shown three different parties in first place - the Lib Dems, the Brexit party and Labour - the first time this has happened since 1986.
But the nature of the First Past the Post (FPP) voting system used at Westminster elections means Labour and the Conservatives could still cling on to hundreds of seats even if their support continues to plummet.
Reformers want the UK Parliament to follow the example of Scottish elections by allowing a degree of proportional representation into how the electoral system.
The Electoral Reform Society (ERS) said voters remained "hampered by a binary, out-dated voting system" with small changes in support likely to mean the difference between parties winning dozens or seats and none at the next election, given the state of electoral fragmentation.
READ MORE: How Scottish Labour found itself in a ‘death spiral’
One recent poll put Labour and the Conservatives in third and fourth place with less than 40% of the vote. The ERS said there was a "clear, long-term trend" that politics was moving away from two main parties.
Commenting on the ERS research, Sir John Curtice, the country's leading pollster, said: “There is little doubt that Britain's traditional two-party system is facing its biggest challenge yet in the wake of the Brexit impasse.
"If that challenge persists it would seem inevitable that there will renewed debate about the merits of the first-past-the-post electoral system.”
Since Labour became the second largest party in terms of MPs at the 1922 general election, the lowest combined Conservative and Labour vote share seen in Great Britain in a general election is 67% in 2010.
The ERS claim the results of the next General Election will be a "total lottery" and likely to be the most disproportionate in history given Westminster’s first-past-the-post voting system.
It argues it is time for Westminster to join Scotland, Wales and local elections in Northern Ireland and Ireland in introducing a ‘fair, proportional voting system’ where seats in Parliament accurately match how people vote.
READ MORE: 'Tories and Labour pay price for ignoring Scotland’s warning'
Darren Hughes, Chief Executive, Electoral Reform Society said: “We are seeing a fundamental shift in our politics with voters turning away from the two main parties in record numbers and supporting a wider range of parties than ever before.
“We now find ourselves a multi-party democracy trapped in a two-party system – a system that is breaking at the seams as our fragmented politics tries to find a place in a one-person-takes-all voting system.
“Because of our broken voting system the next election could see the most disproportional Parliament in our history. Or voters may feel forced to ‘game the system’ by voting for their second or third preference party. Whatever happens, it will be a lottery for voters, with volatile results.
“It’s now more vital than ever the government commits to a fair and proportional voting system in Westminster, one that gives us a parliament that represents the views of the people it serves.”