Scottish National Party Leader Nicola Sturgeon on the campaign trail

As Australia contemplates reforming a Senate electoral system that’s showing considerable wear after three decades of service, it’s worth bearing in mind that Britain will hold an election next week based on a principle that has gone undisturbed since electoral democracy first became discernible there in the 14th century.

But with the nation’s political order under increasing strain from voter disengagement, Scottish nationalism and controversy over its place within the European Union, the virtues of the single-member first-past-the-post system are increasingly being called into question.

Throughout the age of liberal democracy, Britain and its Westminster system have been the exemplar of what Dutch-American political scientist Arend Lijphart identifies as the majoritarian strain of democracy, in which a nation’s sovereignty is vested in a parliament with only one chamber of real consequence, and elections are held on a winner-take-all principle that encourages the development of a two-party system.

The alternative model identified by Lijphart, consensus democracy, is generally associated with countries that have pronounced religious, ethnic or linguistic divisions, which can be accommodated through power-sharing mechanisms such as federalism, strong second parliamentary chambers and elections conducted through proportional representation.

Australia’s system is sometimes faulted for grafting the consensus feature of federalism on to the Westminster model, a contradiction that became explicit when a government with a clear lower-house majority was effectively turfed from office by the “states’ house” in 1975.

Britain, on the other hand, has been spared serious constitutional crises since the House of Lords was put in its place in 1911 — which, in view of the contrast continental Europe offered through much of the 20th century, meant that arguments for serious constitutional or electoral reform failed to gain traction.

However, prognosis for next week’s election offers any number of reasons to shake the country from its complacency.

As of the 1960s, elections in Britain typically involved as much as 90% of the voting public plumping for the Conservatives or Labour, notwithstanding a lingering base of support for the once mighty Liberal Party.

But since then, party loyalties have unravelled to the extent that barely two-thirds seem likely to do so this time, even though the electoral system in most cases offers little hope of reward for those who favour minor parties.

Prior to the last election in 2010, hostility to the established order was evident through the strength of the Liberal Democrats. Founded in the 1980s as an alliance between the old Liberal Party and centrist Labour dissidents, this party had polled at least 17% of the national vote since the Thatcher years, rising to 22% in 2010.

But the Liberal Democrats abandoned their outsider status after the election by entering a coalition with David Cameron’s Conservatives and suffered a clear collapse in support as a result. Far from encouraging a return to the major parties, this has led to a renewed fracturing of the party system.

The Green Party, which has long been handicapped by the electoral system, stands to increase its national vote by an order of magnitude as left-of-centre voters desert the Liberal Democrats. At the other end of the spectrum, UKIP, as the UK Independence Party now brands itself, has enjoyed phenomenal success at byelections by putting a presentably moderate face on Euroscepticism and anti-immigrant sentiment.

But the most portentous realignment has involved the Scottish National Party, which stands to find consolation for the defeat of September’s independence referendum by redrawing the Scottish electoral map.

So far as voting behaviour is concerned, Scotland first developed a life of its own after the Thatcher revolution gutted its industrial and mining sectors, a pattern that was equally evident through much of northern England.

The great breach came when Tony Blair led Labour to power at the 1997 election, at which the Conservatives didn’t win a single one of Scotland’s 72 seats. At both Westminster elections and those for the devolved Scottish Parliament, which was established in 1999, the Conservatives have since found themselves at the margins of a realigned party system dominated by Labour and the SNP.

Now it’s Labour’s turn to face devastation north of the border, with most projections suggesting it will lose all but a handful of its 41 Scottish seats, while the SNP’s tally is set to go from six to around 50.

The short-term significance of this is that Labour has next to no chance of winning enough seats from the Conservatives in England and Wales to secure a majority in its own right.

But perhaps more importantly, it confirms that Britain has become a square peg in the round hole of its majoritarian electoral system — which, in Lijphart’s formulation, is something appropriate only for homogenous societies with a broad consistency of voting behaviour.

With the Conservatives now enduringly uncompetitive outside of England, and Labour apparently set to join them in that distinction in Scotland, it would only be a slight exaggeration to say that both are now structurally incapable of securing the kind of clear victory that British voters have traditionally taken for granted.

This means the presumed virtues of a majoritarian electoral system — decisive government and clear accountability, which in any case have been compromised to some extent by devolution — are no long being delivered, making it increasingly hard to justify its failings of disproportionate seat shares and wastage of votes in safe seats.

Britain has already muffed one shot at reform, when the Conservatives outsmarted the Liberal Democrats in coalition negotiations by agreeing to hold a referendum on optional preferential voting. This went down to a heavy defeat when put to the voters in May 2011, as the Conservatives surely knew it would.

It was thus made clear that in Britain, no less than Australia, significant constitutional or electoral reform would not be a prospect without substantial bipartisan support.

Certainly there is no sign of such agreement at present. But as Martin Kettle of The Guardian argues, that could very well change if the SNP indeed sweeps the board in Scotland, something it can only achieve under a single-member constituency system.

The result could be a SNP stranglehold on the balance of power in the new Parliament — a situation that would prove enormously unpopular in England.

In that event, the cleanest solution short of Scottish independence might well be for Labour and the Conservatives to clip the SNP’s wings by negotiating the introduction of some form of proportional representation, to be followed by a fresh election.