Electoral system could not long survive a perverse outcome in which first party comes third, and third comes first

Small-c conservatives often argue that before a reform is introduced it should be tested not only against normal operating conditions but also against unlikely possibilities, because these can produce unintended consequences.

The first-past-the-post (FPTP) electoral system is capable of spectacular malfunctions, particularly when there are more than two parties in contention for the lead. YouGov, ComRes and Bpix this weekend showed four-point spreads between first and third parties, and election outcomes that seemed extreme must now be considered.

The Bpix numbers for the Mail on Sunday (Lib Dems 32, Tories 31, Labour 28) would produce a bizarre result on uniform swing. The largest party in votes, the Lib Dems, would come third in seats with 120, and the smallest of the three main parties, Labour, would win the largest number of seats – 268. The Tories would have 230.

If we transpose the Conservative and Labour Bpix percentages, then despite the Lib Dems being the largest party on 32%, Labour with 31% would have 323 seats in parliament. While not quite an overall majority, it would be an effective majority given the Speaker's neutrality and Sinn Féin MPs not participating.

The reason for these peculiar possibilities is to be found in geography. Labour is resilient to a falling national share because it has an efficiently distributed vote – a large number of low-turnout strongholds, competitive marginals and few votes in its high-turnout hopeless seats.

But because Lib Dem support is fairly evenly spread, it is hard for that party to translate broad increases in its share of the vote into seat gains, and the gains it does manage tend to come from the Conservatives.

The Lib Dems received respectable 20-30% losing shares in many constituencies in 2005, and a 10-point increase across the board would merely produce impressive 30-40% losing scores in most of these seats.

The electoral system could not survive a perverse outcome in which the first party comes third and the third party comes first – or one in which the second-placed party has an overall majority, despite the support of fewer than one voter in three. Either case would make Florida in 2000 look like a model of democracy. There would be a justified crisis of confidence in a political system that had produced such a travesty.

Gordon Brown's preferred reform of the alternative vote (AV) would help the Lib Dems win more seats under AV than FPTP. Rather than being unfairly penalised in a near-even, three-way split of votes, the Lib Dems would hit the jackpot. If three party politics is here to stay, Labour would be best advised to modify its position on AV, and go instead for a proportional system based on related principles, namely single transferable vote (STV) or AV-plus, which both involve multi-member constituencies. But the prime minister does not seem ready to get rid of single-member seats.

The Conservatives' problem is potentially worse. Their atavistic love of FPTP could surely not survive such a refutation of the claim that it gives stability and a clear choice of government.

Even if a single-party government emerged on the basis of 32% support and a one-point lead – or deficit – in votes, it could not claim a "mandate" without provoking mocking laughter.