General elections in the UK used to be about two parties. Labour and the Conservatives. They used to get most of the votes between them. A few percent would go to the third party and "others" but the vast majority voted red or blue.
In this sort of situation, where you effectively have a two party system, the First Past the Post electoral system was perceived as working quite well. It often ensured that one of the parties had a majority (even though this was often on a minority of the vote) but it was often only a few percent difference and usually the party that won the election in terms of seats, got the most votes (although not always - February 1974 being a good example of this).
In 1951, 93.1% of voters plumped for red or blue. In 1955 it was 96.1%. The problem is that since the 1950s, the proportion of people who vote for one of the two main parties has declined. Here is a table showing the shares each party got for each election from 1951 - 2005 and the combined total (source - collated from Wikipedia pages on general election results):
As you can see, we have gone from 90 odd percent voting for Labour or Conservative in the 1950s to only around two thirds voting for both of thoise parties in 2005. Here is the data in graph form which makes the decline a bit clearer:
Now, before anyone accuses me of hypocrisy, I am well aware that I have complained before about graphs that don't start from 0 but there is a reason I have done it like this. Frankly, if support for the two main parties combined at a general election ever dropped below 50% then it would be game over for First Past the Post irrespective of whether (or probably even because) one of the two parties could form a majority government. This is a psychological floor to my argument which is why I have made it the bottom of the graph.
As can be seen from this graph, there is a bumpy but steady decline. You could draw a line of best fit through this which if continued would suggest that in another 20 years or so we could well hit that 50% mark.
As if to prove my point, looking at the latest "General Election Prediction" from Electoral Calculus which analyses opinion polls and electoral geography to come up with their best bet of how the votes will go at the next election their current prediction (from 7th July 2009) is:
So a combined predicted total of 62.34%. This would follow the line of best fit from the above graph perfectly showing a continuation of this trend of decline in support for the two major parties.
So what conclusions can we draw from this? I would say that we are becoming increasingly pluralistic as an electorate. We have got used to voting for smaller parties and those often described as "others". I think the vote share of the two main parties will continue to decline and it will get harder and harder for them to justify the present system.
At the moment, those in favour of First Past the Post have the power of incumbency with it being the existing system and inertia will generally militate against a change. However recent opinion polls have shown that people are generally in favour of a more proportional system and as that line of best fit comes down and down over the next few years I think things will flip. Proponents of FPTP will be pressured to justify why such a manifestly unfair system that disproportionately benefits Labour and the Conservatives should be allowed to continue.
UPDATE: Tom Freeman has done an excellent follow-up to my post here where he hypothesises what could happen if the vote shares for Tory and Labour gradually came down and the others gradually went up. He's got pie-charts and everything!