Last year's general election result was determined by less than 2% of voters, according to a think tank report which has denounced first-past-the-post voting as a "broken system" for choosing Westminster MPs.
The report from the left-of-centre Institute for Public Policy Research comes ahead of the May 5 referendum on replacing first-past-the-post (FPTP) with the alternative vote (AV) system, under which voters rank candidates in order of preference.
While supporters of FPTP - including most Conservatives and a large number of Labour MPs - argue it is the best way of producing strong governments, today's report argues that long-term changes in voting patterns mean the system has become a recipe for hung parliaments and coalitions in future.
Only about 31% of voters - 9 million people - live in the marginal seats which form the main battlefields in elections under FPTP, found the IPPR. But the number whose votes made up the margin of victory in seats which actually changed hands was even smaller - just over 460,000, or 1.6% of the electorate.
The figures meant that 69% of votes cast last May had little chance of making any difference to the result, while more than 98% did not make a decisive impact, said the think tank.
More than a third (34.9%) of votes in the 2010 general election were cast for parties other than the "big two", increasing a trend which has seen Labour and the Tories together garner a dwindling share of support, found the report.
While the two biggest parties between them secured 96.8% of the vote in 1951, that share has now fallen to just 65.1%.
The increasing success of smaller parties makes single-party government less likely, with the major parties now requiring 85 more seats than their major rivals to secure power and 100 more to enjoy the kind of parliamentary majority needed to govern effectively.
Even if Liberal Democrat representation fell from 57 MPs to 25 at the next election, the victorious party would need to return around 50 more MPs to Westminster than their main rivals to be confident of an outright majority.
And the report also highlighted the inbuilt bias to the FPTP system which means Labour can win an absolute majority with a 3% lead over Tories, while the Conservatives would need an 11% advantage - leaving a 14% gap in which a hung Parliament is the most likely result.