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The 2015 general election was the "most disproportionate in British history", the Electoral Reform Society has said.

In a new analysis the society - which campaigns to change the voting system - has assessed how the make up of Parliament would have differed had other voting systems been used.

The research shows UKIP could have won as many as 80 MPs and the Greens 20.

UKIP received 3.9 million votes and the Greens 1.2 million, and they ended up with one MP each.

'Fairer system'

Katie Ghose, chief executive of the society, described the current system, usually known as first-past-the-post, as "archaic" and "divisive".

She said: it "leaves millions disenfranchised and forces millions more to feel that they have to vote for a 'lesser evil.'"

"It's about time we had a fairer system for electing our MPs."

How different voting systems work

First-past-the-post (used in UK general elections): People get a single vote for who they want to represent their constituency and whichever candidate gets the most votes wins.

Single transferable vote (used in Northern Ireland Assembly elections and Scottish council elections): Voters number candidates in order of preference and all those passing a defined threshold - calculated by dividing the number of valid votes by the number of seats plus one - are elected. Their surplus votes are distributed to other candidates on the basis of other preferences with low-scoring candidates being progressively eliminated. The system sees more than one candidate elected from a single constituency.

Additional member system (used in Scottish Parliament and Welsh Assembly elections): Some representatives are elected via the traditional first-past-the post method but voters get to cast a second vote for "top-up" seats, allocated in proportion to the number of votes. These representatives are selected on a regional basis from lists of candidates drawn up by each party.

Party list system (used in European Parliament elections): The UK is divided into large constituencies, known as regions, and different parties put together lists of candidates for election, with their preferred choices at the top. Seats are allocated, on a top-down basis, in proportion to parties' share of the vote.

Supplementary vote (used in London mayoral elections): Voters choose their first and second preferences and a candidate can only be elected in the first round if they get 50% of the vote. If no-one achieves this, all but the top two candidates are eliminated and their second preferences redistributed to the candidates still in the race. The candidate with the most votes is then elected.

Alternative Vote (rejected in 2011 UK-wide referendum): Voters rank candidates in order of preference and anyone getting more than 50% in the first round is elected. If that doesn't happen, the candidate with the fewest votes is eliminated and their second choices allocated to the remaining candidates. This process continues until a winner emerges.

Alternative Vote Plus: As well as voters ranking constituency candidates in order of preference, they would get a second vote at a regional level either for a party or for their favourite candidate from a list proposed by the parties. This would mean having a group of constituency MPs and a group of "party list" MPs.

'Breaking up Britain'

In a survey commissioned by the society and carried out by pollsters YouGov, over 40,000 people were asked how they would have voted in the general election had they been required to rank the parties in order of preference.

The report, entitled The 2015 General Election: A Voting System in Crisis, found that under a list-based system of proportional representation similar to the kind used in European elections, the outcome would have been very different.

In this case Conservatives would have won 242 seats (-89), Labour 208 (-24), the SNP 30 (-26), the Lib Dems 47 (+39), Plaid Cymru 5 (+2), UKIP 80 (+79) and the Greens 20 (+19).

First past the post v List PR

However, the analysis also found that under analternative vote system, where voters' preferences are reallocated until one candidate gets over 50%, the election result would be similar to that of 7 May, with the Conservatives winning 337 seats - an increase of six.

In this case Labour would have 227 (-5), the SNP 54 (-2), the Lib Dems 9 (+1), Plaid Cymru 3 (no change), UKIP 1 (no change) and the Greens 1 (no change).

First past the post v Alternative Vote

And the research showed that under a single transferable vote (STV) system, similar to the kind currently used in Scottish local elections, the Conservatives would have won 276 seats (-55), Labour 236 (+4), the SNP 34 (-22), the Lib Dems 26 (+18), Plaid Cymru 3 (nc), UKIP 54 (+53) and the Greens 3 (+2).

First past the post v STV

"Our voting system is breaking up Britain," Ms Ghose added.

"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."

The report also found:

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50% of votes in the election - 22 million - went to losing candidates

2.8 million voters were likely to have voted "tactically"

The election saw an MP win on the lowest vote share in electoral history 24.5% in Belfast South

331 of 650 MPs were elected on under 50% of the vote, and 191 with less than 30% of the electorate

Since the election, politicians from UKIP, the Greens, Lib Dems, Plaid Cymru and the SNP have supported a petition calling for voting reform.

The Electoral Reform Society used the D'Hondt method for converting votes to seats in a list-based PR system.

Ms Ghose later told BBC Radio 4's Today programme: "Voters have changed fundamentally. They're shopping around, they're supporting a wider range of parties than ever before and I think that the system's really struggling to cope with that."

But Labour MP Dame Margaret Beckett said the current system tended to give the electorate what they wanted.

"One of the virtues of our present system is that the British people understand it, they know how to work it. In 2010 they didn't like any of us and they didn't give any of us a majority," she said.

"But in 2015 they said 'hang on a minute, we'd rather have a majority government of one or the other, than a mess'."