Labour's election victory on the lowest share of the vote has fuelled fresh calls from campaigners for proportional representation to be brought in.
Campaigners say the current system is unfair
Tony Blair won a Commons majority of 67 despite Labour winning little more than a third of votes nationwide.
The Conservatives won the biggest share of votes in England but secured nearly 100 fewer English seats than Labour.
PR campaigners say the system is a "travesty" but its supporters argue it ensures stable government.
New Liberal Democrat MP John Hemming on Tuesday led a small protest to Downing Street, with campaigners wearing gags to demonstrate against the unfairness of the existing system.
The UK uses a first past the post voting system which means a candidate simply has to win the most votes in their constituency, not a majority of the votes.
A proportional representation (PR) system would aim to elect MPs using the share of the votes cast for each party.
The election saw Labour win 36% of the votes across the UK but 55% of the seats in the House of Commons.
VOTES FOR GOVERNMENTS
1992: 14,093,007 (41.9%) for John Major's Tories
1997: 13,518,167 (43.2%) for Tony Blair's Labour
2001: 10,740,646 (40.7%) for Labour
2005: 9,556,183 (35.2%) for Labour
The Lib Dems secured 9.5% of the Commons seats on their 22% share of the national vote.
In England, the Tories won 35.7% of the votes compared with the 35.4% won by Labour.
But Labour has 286 English seats and the Tories 193.
Labour peer Lord Lipsey, chairman of the Make Votes Count campaign group, asked how any government could claim to have a valid mandate when it was backed by only 36% of the 60% of possible voters who turned out.
"The British first-past-the-post electoral system has reduced the general election to a travesty of democracy," he said.
Alex Folkes, from the Electoral Reform Society, said the current system was biased against the Conservatives.
"Boundary changes that are due will help to correct some of that but not all," he said.
"We think the Conservative Party would do well to accept that probably the best way of them getting back into power in the immediate future is through a system of PR."
Lib Dem leader Charles Kennedy has said Labour's victory on a record low vote share raises "serious questions for democracy".
He says his party must return to the case for a fairer voting system in the wake of the election system.
The Lib Dems want larger constituencies electing six MPs on the basis of the share of the votes.
They would also use a single transferable vote system where voters put candidates in order of preference. If no candidate wins a majority of first choice votes, the second preference votes are counted.
The Conservatives have opposed PR in the past and their opposition does not look set to shift.
New Conservative MP Greg Hands said: "The system works, looking back over the last hundred years, Britain has had a good stable democracy, characterised by stable sometimes strong governments.
"That is what the system is designed to encourage."
Labour's election manifesto says the party will review the different election systems used in the European elections and polls for the Scottish Parliament and Welsh Assembly.
It says there should be a referendum on any proposed changes to the voting system for Westminster elections.
Mr Blair appointed a royal commission, headed by Lib Dem peer Lord Jenkins, to examine electoral reform in 1998 but no fundamental changes have followed.
The report proposed a system called Alternative Vote Plus, where as well as constituency MPs, there would be a top-up list to ensure a more accurate reflection of total votes cast.