Then again, the history of political parties' interest in electoral reform shows its time has frequently come - and gone away again just as often after they win power under the old system.

The undeniable link between a party's intent to reform and its parliamentary strength is one of the few constants in the shifting debate on changing the system voters use to their send their MPs to the House of Commons.

Getting keen, cooling off

In its earliest years, the Labour Party was largely in favour of proportional representation (PR). For Keir Hardie and many of the party's founders it provided a solution to Labour's reliance on the secret electoral pact with the Liberals at the start of the century.

But as Labour started to win more seats at Westminster, so its enthusiasm for PR dimmed. The idea of electoral reform disappeared from the party's agenda altogether after the 1923 election when Labour won a larger percentage of seats than of votes, and formed a minority government for the first time.

Rather than change the first-past-the-post system, Labour under Ramsay MacDonald instead now saw how it was possible to marginalise its Liberal opponents with it.

At around the same time, and by no small coincidence, the Liberals began to see PR as a vital issue. It was, they decided, essential to their own survival. Ever since, PR has been a central theme of their policies - as seen with today's Liberal Democrats.

Shifting positions

The Tories won more votes, but Harold Wilson became prime minister in 1974

Debate on the subject gently simmered away over the following years, but it was not until the mid-1970s that it sparked back into life in the political mainstream of the dominant parties.

This time it was the Conservatives who discovered a new and sudden interest in the prospect of electoral reform when at the February 1974 election the party won more votes than Labour - yet fewer seats, and Harold Wilson became prime minister.

The pressure group Conservative Action for Electoral Reform was formed within the Tory Party, and counted among its supporters such future heavyweights as Douglas Hurd and Chris Patten.

Michael Ancram, at the time a new Tory MP, was another who backed PR. The man who is now Conservative Party chairman has more recently been laying into the government for attempting to "rig" the electoral system by considering PR for Westminster.

Chris Patten supported PR - after the Tories lost the election

Once the Tories were returned to office with a healthy majority in 1979, the internal push for electoral reform evaporated.

Now it was Labour's turn to become seriously taken with the idea once more - though not until the late 1980s.

Complicated by party splits . . .

It might have happened earlier were it not for the fact that at the start of the decade, PR became associated with the hated defectors - including Roy (now Lord) Jenkins - to the breakaway SDP. Changing the electoral system became the single policy most closely identified with the new party.

But having lost its third general election in a row in 1987, the mood within Labour quickly changed. The Tories had, as in 1979 and 1983, again won a large parliamentary majority on less than 45% of the vote.

Tony Blair appointed Liberal Democrat Lord Jenkins to chair the voting system commission

Electoral reform became highly fashionable with the left-leaning intelligentsia in and around Labour. In 1988 the constitutional reform pressure group Charter 88 was launched with PR for the House of Commons one of its chief objectives. Inside the party, the Labour Campaign for Electoral Reform enjoyed a surge in support.

The Labour leadership was not immediately impressed. Indeed, then-Labour leader Neil Kinnock famously dismissed Charter 88 as chattering class "whiners and whingers". Within a few years he had joined them as a fully paid-up member.

. . . And splits within parties

Labour's new enthusiasm for electoral reform soon had an effect on party policy. Elections to the proposed Scottish Parliament could not, it was decided, be held under the first-past-the-post system.

Labour also established a special committee, chaired by the academic Raymond (now Lord) Plant, to examine electoral systems for everywhere except the Commons.

Neil Kinnock became a convert to the cause, John Smith remained unconvinced

After receiving an interim report from the Plant Committee, Labour adopted a cautious "wait and see" approach in the run-up to the 1992 election.

Proportional representation was still a highly contentious issue within the party. Neil Kinnock had become privately converted to the PR cause. His deputy, Roy Hattersley, remained vehemently opposed. The shadow cabinet was divided, and the pro- and anti-reform camps straddled the party's left and right wings.

After Labour lost the election, a second Plant report was delivered - this time in favour of reforming the way MPs were elected.

Labour's new leader John Smith remained unconvinced of the case for changing the electoral system for the Commons.

He kicked the report - and the inevitable wrangling a policy statement on the subject would have caused - into the long grass by pledging a Labour government would hold a referendum on the issue in its first term.

Blair's inheritance

Tony Blair entered Downing Street having inherited a party split on electoral reform

This is what Tony Blair inherited in 1994. Though in favour of retaining first-past-the-post, the current Labour leader is in the same position as his predecessor: he leads a party that is, from the grassroots to the cabinet, across left and right, split on the issue.

One important difference is that Mr Blair is now prime minister with the biggest majority the party has ever enjoyed - all on less than 44% of the vote.

In keeping with the tradition of political parties' warmth towards PR turning lukewarm with the onset of power, Labour's party conference this year saw a number of ministers arguing that perhaps the pre-election promise of a referendum during this parliament did not, after all, mean quite what everyone thought it did.