British democracy is flawed at its heart. A voting system that was barely defensible in an era of broad, stable class loyalties has become grotesquely unfair as those loyalties have eroded. It now destroys our engagement with politics and damages the integrity of MPs. It helps give the British state its particular character – the peripatetic flow of initiatives from a controlling centre and its symbiotic relationship with a hyper-populist media. It creates parties in thrall to vested-interested groups, mostly recently Big Finance.

Above all, if fairness is a value we care about, the voting system is an offence to any conception of fair political representation. A state that can collude in this degree of unfairness in its electoral system is unlikely to be especially fair itself.

It was hardly democratic in a former age when 95% voted for two parties. With the TV debates creating a genuine three-party race, the probable distribution of seats in a new parliament makes first-past-the-post untenable. The problem is not the lack of a working majority, it's that Labour could lose the election but in effect win the parliament.

While the Liberal Democrat vote is evenly spread, Labour's vote is concentrated in the urban and old industrial constituencies in the Midlands, the north, Wales and Scotland while Conservative votes pile up in the rolling shires. If there needs to be a coalition government, negotiating it becomes infinitely harder when the parliamentary power of the parties departs so incredibly from the political reality outside.

The charts here, drawn from information on the BBC's seat calculator, illustrate just how unfair the results could be. If the votes for the three main parties stood at 30% each on May 6 (with "others" such as the Scottish and Welsh nationalists at 10%), Labour would return to Westminster with 315 MPs, 11 short of an overall majority, while the Conservatives have 206 and the Liberal Democrats a mere 100.

In a second outcome, with the Lib Dems topping the poll with 34%, the Tories coming second with 33% and Labour last with 32%, Gordon Brown's party would retain 300 seats. More extraordinarily still, if the Tories win the popular vote with 33%, the Lib Dems come second with 30% and Labour last with 27%, Labour would pip the Tories to be the largest party in the House of Commons by 262 to 257 – with the Lib Dems picking up a lowly 102 seats. Britain sends observers to elections in countries which have fairer voting systems. Defenders of the status quo have to argue that this is a one-off, transitional, if admittedly bizarre political outcome, the result of the progressive part of the country deciding which of the two broadly liberal parties is going to be its chief standard bearer.

Once that is settled, there will be a new duopoly between the Conservatives and whatever dominant progressive party emerges – either a reconstructed, rejuvenated Labour party or the newly mature Lib Dems. Politics in the conception of those who push for the current system is a grand game between two broad opposing coalitions of left and right who seek a mandate for "firm" government with a first-past-the-post system. The voting system is only playing back to us, albeit in a vastly exaggerated way, the political truth that both the current Conservatives and Labour parties are malfunctioning political constructs. They say that once the new left and right formations are reconstituted, which will take time, first past the post will work well.

The argument does not work, and on so many levels. In the first place it presupposes that democracy is only about grand left/right clashes and government can only be successfully prosecuted by one political party in control. Neither is true. Democracy is the process that anchors the public realm, but it is the vitality of the whole public sphere that allows both good government and democratic argument. Is government effective and well-co-ordinated? Are official decision-makers held properly to account? Are our parties and the government machine too easily captured by vested interests? Do citizens feel the complexity and range of their views count and are even heard? What is the quality of public debate?

On all these scores Britain does not do well. The British electorate is changing. We are better educated. Our political loyalties are much more fluid. We want our votes and opinions to count. Yet the majority of us vote in constituencies which are essentially rotten boroughs that will return the same party come what may. Your choice, if not from a winning party, is not to vote or vote tactically. Only in the 100 or so marginals, where Lord Ashcroft has directed his millions, is there a genuine political contest. It is the fast road to voter disengagement.

Bidding for votes from this fluid, much more biddable electorate becomes a life and death game in a first-past-the-post system. If you can move a few per cent towards you, especially in the marginals, control of the state falls into your hands. With stakes this high party leaderships impose iron control on their parties to stay "on message". They court the good opinion of the aggressively populist media with populist initiative after populist initiative. The centre becomes ever more important. Political debate and argument is behind closed doors, with MPs rehearsing the line to take and voting dutifully as lobby fodder. A political career for many MPs is reduced from being an honourable vocation to being the puppet of the small coterie around the leader – as true of David Cameron's and Gordon Brown's teams today as it once was of Tony Blair's.

The parties are much more open to the rich and powerful – whether trade unions in the past or now Big Finance. In a first-past-the-post system like ours it is not the views of the party members and MPs that count, but the leadership who control and dictate so much. Capture the leadership's ear with the promise of party donations, threats of exiting the country if you don't get what you want, or assurances of compliance in some dearly held party aim and you can have enormous influence – as a succession of investment bankers, private equity and hedge-fund partners have found with all three parties.

Proportional representation does not end all this; Big Finance will be still be here, if hopefully much reformed. But it changes democratic dynamics. It opens the system up. It forces argument out into the public domain. It connects party leaderships with their MPs, and MPs with their voters. It requires the voter to be engaged and serious about politics. It will embolden some politicians to be less in thrall to the right-of-centre press. Above all, proportionality is at the heart of fairness.

To move to a proportional voting system is a huge step; it asks our politicians to redefine what they consider fair. The fairness in first past the post is the fairness of the primitive hunter-gatherer or the landed aristocracy: I eat all that I kill and leave only scraps; I own all that I survey; I accept no claim by others nor enter into any discussion. I and my party won this election. We have a majority in the House of Commons. The state is ours. We will now feast, at least until we worry about the next election. This, apparently, offers the firm government that Britain needs.

This is nonsense. The world is too complex and the challenges too difficult for one party to claim a monopoly of right – and to set about governing to dictat, while managing a 24/7 media with an endless flow of populist initiatives and offering private concessions to powerful vested interests. The Labour party manifesto has made the tiniest of moves towards changing our voting system. One good outcome in the next House of Commons is that only via a coalition with the Lib Dems could Labour command a majority – and the Lib Dems will insist on more proportional voting as the price of their support. On the other hand, if the Conservatives were to win, they would entrench first past the post – but with 10% fewer constituencies. Thus, they hope, they would guarantee Tory majority rule for decades – and secure the character of the state.

But that is the heart of our problem as a country. Even Tories are beginning to realise the current voting system is unsustainable. The Lib Dem insurgency is serving the country well. This is becoming an epic and crucially important general election.

• More election comment from Cif at the polls