In its early days, the Labour Party, unlike almost all of its social-democrat counterparts in western Europe, was hostile to proportional representation. That was largely due to the first Labour prime minister, Ramsay MacDonald, who was secretary to the Labour Representation Committee from the time the party was established in 1900.
MacDonald was playing a long game, looking forward to the time when Labour would become a major party in the state. It would then benefit from the exaggerative effects of first-past-the-post (FPTP), which yields a heavy bonus in seats to large parties.
George Lansbury explicitly revealed Labour's thinking when he told the 1926 Labour party conference that, speaking personally, he thought that most of the decisions under the present system had worked for other people but, if Labour was wise, it could now make the system work for the party itself.
It is perhaps debatable how far the party leaders succeeded in making the system work in Labour's interest, as the 20th century proved to be the "Conservative century". Between 1914 and 1997, there were just two general elections at which Labour achieved a working majority - 1945 and 1966. It was only in the years between 1945 and 1950, and 1966 and 1970, that the left could do things that the Conservatives seriously disliked. One explanation for Conservative hegemony lies in the division on the left between Labour and the Liberals.
However, since 1997, there has been a sea change in Labour attitudes. In that year Tony Blair established the Independent Commission on the Voting System, chaired by Roy Jenkins, to consider alternative methods of election. It reported in 1998, advocating proportional representation. Blair promised a referendum on reform but without giving a date. He was notoriously uninterested in constitutional issues, though his government will be remembered in part for the major and probably irreversible constitutional reforms that it instituted.
Blair's attitude towards proportional representation was always highly pragmatic. What he wanted to achieve was party realignment - a coming together of Labour and the Liberal Democrats to re-create the progressive alliance that ruled Britain before 1914. If proportional representation could contribute to that aim, then he would be in favour of it.
The landslide majorities achieved by Labour in 1997 and 2001, albeit on 42 per cent of the vote, put paid to notions of realignment. But at the Labour party conference in September, Gordon Brown revived the idea of reform, promising a referendum on the alternative vote system shortly after the general election if Labour is returned. The alternative vote is not a system of proportional representation - indeed, it can yield even more disproportional results than first-past-the-post - but perhaps this matters less than the fact that Labour is in the process of committing itself to change. The Conservatives are now the only major party still in favour of first-past-the-post, even though the system is heavily biased against them -a real triumph, perhaps, of principle over self-interest.
Meanwhile, as a largely unnoticed by-product of the era of constitutional reform, it has become accepted that elections to bodies other than the House of Commons or English local authorities should be by proportional representation. It was broadly agreed that, were there to be devolution to Scotland, Wales and Northern Ireland, the devolved bodies should be elected by proportional representation. That was a quite unexpected development because, until very recently, it was generally assumed that first-past-the-post was natural to Britain. Opposition to it was confined largely to the Liberals and their arguments could be easily dismissed as special pleading. But there are now no fewer than four electoral systems in operation in Britain in addition to first-past-the-post.
So it seems that proportional representation is rather like the incoming tide, flowing into the estuaries and along the rivers. It is becoming difficult, if not impossible, to hold it back.
The electoral systems used in Scotland and Wales allow opinion to be reflected directly in the legislature, in accordance with its strength among voters. As no party has been able to win a comfortable overall majority, the outcome has been coalition or minority government. For most of the time, Labour has been in coalition with the Liberal Democrats, but in Wales, since 2007, it has been in league with Plaid Cymru. These coalition governments have not hindered progressive politics in Scotland and Wales. Indeed, they may even have facilitated it.
In 2007, the Scottish National Party (SNP) became the largest party in Scotland. Under first-past-the-post, it might well have won a majority of seats in the Scottish Parliament in a four-party system on the 33 per cent of the vote that it gained. However, proportional representation has obliged it to work with other, unionist, parties. So, the reformed system has become a mainstay of the union between England and Scotland, making it impossible for the SNP to win an overall majority in Holyrood unless it also has the support of a majority of Scottish voters.
Elections to the devolved bodies reflect opinion in Scotland and Wales. By contrast, elections to the House of Commons distort it, masking, in particular, the decline of two-party politics since the 1970s. In the eight general elections between 1945 and 1970, the average share of the two-party vote was 91 per cent. In the nine general elections since, it was just 73 per cent and, in 2005, the two-party share fell to 69 per cent. Nearly one-third of those voting refused to support either of the major parties.
Since February 1974, the share of third and other parties has never fallen below 20 per cent. Between the general elections of 1945 and 1970, there were just ten MPs on average from parties other than Labour and the Conservatives. Since 1970, the average has been 54. In the 2005 parliament, no fewer than 92 MPs came from parties other than Labour or the Conservatives, nearly 15 per cent of the total.
The reason that the decline of two-party politics in the country could go virtually unnoticed was that Britain was still ruled by single-party majority governments. The electoral system manufactured landslides for parties with just over two-fifths of the vote - for Margaret Thatcher in 1983 and 1987, for instance, and for Tony Blair in 1997 and 2001.
In 2005, Labour gained a comfortable working majority of 67, although its vote had slumped to just 36 per cent, the lowest percentage vote ever recorded in modern times for a majority government with a secure majority. Labour was returned to office even though nearly two-thirds of those voting had voted against it. There is a disjuncture between the two-party system at Westminster and a multiparty system in the country, a disjuncture between popular attitudes and their representation in the Commons.
The Westminster culture and the electoral system that sustains it are, in essence, a product of tribal politics. This culture was at its strongest and most unquestioned when tribal politics was at its height, the years 1945 to 1974, now misleadingly seen as a "norm" from which politics has since deviated. At the height of this period, in 1951, the two main parties, Labour and the Conservatives, gained 97 per cent of the vote between them. There seemed no serious dissatisfaction with the British political system.
Today, however, the rise of proportional representation may be seen as a practical response to the decline of tribal politics, allowing - even requiring - rival parties to co-operate. The forms of government that operate in Scotland and Wales are more suited, surely, to the modern age than is the adversary culture of Westminster, which remains mired in the politics of yesterday.
In the 1980s, the Liberal leader David Steel argued that electoral reform would be the key in the lock, opening the door to wider constitutional reform. However, constitutional reform has occurred without electoral reform at Westminster.Electoral reform would crown the edifice, completing the era of constitutional reform by finally putting an end to the culture of tribalism. With electoral reform, Britain would have not only a fairer electoral system, but a chance of making the 21st century, unlike the 20th, the "progressive century".
Vernon Bogdanor is professor of government at Oxford University. His book "The New British Constitution" was recently published by Hart (£17.95)