What if ... the voting system was fair?

In British politics the killer argument against abandoning the first-past-the-post (FPTP) voting system has been that, however unfair it might be to smaller parties, it produces strong stable majority government by a single party. Not so in 2010. But the argument's proponents appear to have been vindicated with a vengeance in 2015. David Cameron returns to Downing Street with a Commons majority although he has secured a national vote of only 37 per cent. That's how they define "legitimacy".

But how would a truly proportional system, in which MP numbers reflect voting shares, have worked this time out? If we apply strict proportionality Cameron, with a notional 234 MPs instead of 331, would still have achieved a plurality – more seats than any other party – but would have had to build a difficult majority in a very different PR-elected Commons. (In practice it should be noted that Ireland’s STV system with small multiple-seat constituencies actually produces less than perfect proportionality).

Ruling out alliances with Labour or the SNP, Cameron would today in such a theoretical Commons need a rampant Ukip's backing (13 per cent and 84 seats) just to achieve a majority. He would probably also want the LibDems (8 per cent) on board to be sure, to be sure, to provide a measure of political balance. A most peculiar animal, that coalition! But Labour on the percentages achieved would find it impossible to find bedfellows for an alternative coalition.

Following the solid majority (68 per cent) against electoral reform in the LibDem inspired referendum in 2011 on a form of PR, such a reform scenario is certain not to appeal to the Tories as the basis for a further attempt to reform the electoral system although the "injustices" thrown up in this election have already prompted calls, unsurprisingly from Ukip and the LibDems, to put it on the agenda.

Irony of ironies, the Scottish National Party, supporters of PR, reaped a magnificent FPTP bonus – the system rewards strong local support while penalising support that is evenly spread across the country – by taking 56 out of 59 Scottish seats with only half of the Scottish vote.

At the other end of the scale the disproportionality of under-representation is far more shocking. The LibDems lost 45 seats to return eight MPs – 1 per cent of seats on a national vote of 8 per cent. As for Ukip, its 13 per cent of the vote garnered representation in Westminster of just one MP, when a strict PR system would have given it 84.

In the 1950s and ’60s, when the Tories and Labour won more than 90 per cent of the votes and of the seats, election outcomes were highly proportional. Since the early 1970s and the rise of the Liberals and others elections have been far less so. The 2015 election has produced the least proportional outcome since the establishment of universal male suffrage in 1918. A heavy democratic price to pay for single party rule?