Our electoral system is no longer creaking: it’s disintegrating around us, showering the political elite with occasional bits of rubble. Lord Gus O’Donnell is no firebrand radical, he’s a crossbench peer who used to head the civil service, but he knows the current political system rests on rapidly shifting tectonic plates. Whatever happens next week, a decisive result is almost impossible, further undermining the legitimacy of an increasingly absurd electoral system.

First-past-the-post was supposed to deliver stable, majority governments: that’s its whole USP. To deliver one full-term hung parliament might be written off as a quirk, an accident: but two begins to look like a trend. If the electorate delivers a left-of-centre majority in parliament next week and Labour assembles a government, its own legitimacy will be under relentless assault by an increasingly unhinged rightwing media. It must surely respond with a referendum on proportional representation.

The signs of morbidity in our electoral system are becoming impossible to ignore

Oh come off it, some will object. You had your referendum on electoral reform a few years ago. It was decisively rejected by the electorate, burying the issue for a generation. But like most of those who took part in 2011, I voted against the option on the table, the alternative vote system – which would have allowed voters to rank candidates in order of preference. This wasn’t because I thought it didn’t go far enough. All electoral systems are a compromise when no party wins more than half the vote. My view was that AV would hand disproportionate influence to the third party – then the Lib Dems, now Ukip – because Labour and the Tories would have to compete for their second preference votes. And like many Britons following the Clegg-Cameron love-in, I believed coalitions involved undemocratic stitch-ups behind closed doors, where governments were assembled that no one voted for and election promises could be easily ditched in the name of compromise.

Such an objection is now redundant. If we’re going to end up with hung parliaments whatever happens, we might as well have representative hung parliaments. Our electoral system is designed for the hegemony of two parties – well, two and a half parties at most. But while nearly all voters opted for Labour and the Conservatives in the 1950s, the combined share is down to about 65%. In Scotland, Labour face wipe-out at the hands of the SNP; in England and Wales, we’ve witnessed the rise of Ukip and the Greens. The two-and-a-half party system has artificially been kept alive, in stasis, by the electoral system, but now the signs of morbidity are becoming impossible to ignore. The political system is increasingly colliding with the reality of political fragmentation.

Couldn’t it just all revert to how things were? It’s possible, I suppose, but that’s surely deeply unlikely. The Tories once amassed votes in Scotland and much of northern England where they are now permanently relegated to second-party status; their membership peaked in the 1950s with nearly 3 million, and is now down to a paltry 134,000; the old religious sectarian divisions they benefited from have diminished. Labour’s support is complicated by the halving of trade union membership, the shift from an industrial working-class to a more insecure and fragmented service sector working-class. Today’s political complexity is simply a reflection of a diversifying social base.

It surely makes sense for Labour to offer a referendum on proportional representation. It would help win over Greens who resent tactical voting, but could be persuaded on the basis of a Labour-led government that could forever banish wasted votes. It would rebut the growing attacks on the democratic legitimacy of a Labour-led government that hasn’t even been formed. It would show a farsighted rejection of political tribalism. Labour will not do so now, because they believe it will only increase media-fanned fears of the influence of smaller parties. But if Labour come second in seats and form a government, they may find that such a referendum is the only way of deflecting a media-led assault. If the minor parties were political savvy, they would offer support only if such a referendum was forthcoming. First-past-the-post lives to fight another election. Who knows, though. It could well be its last.