Fifty years ago , the UK had two big political parties, Labour and the Conservatives, each of wihch were supported by about 50% of the population.
That situation no longer holds, fewer people feel alliegasnce to either of the big parties, and in the last nationwide elections held under PR — the 2004 European elections — Labour and the Conservatives got less than half the vote between them.
We now live in a multi-party society. And while FPTP might have made sense half a century ago, it doesn’t now:
Academics talk about a thing called the “effective number of parties.” In the UK, we have an ENP in Parliament of 2.5 but an ENP in terms of vote share of 3.6. That is an alarmingly high missmatch and as the disparity increases the chances of no-overall control increases accordingly. If the ENP in terms of vote share reaches 4, according to Josep Colomer anyway, “maintaining a majority rule electoral system would be highly risky for the incumbent ruling party” – essentially they lose any real claim of having a mandate (see Helen Margetts’ chapter on Electoral Reform in Unlocking Democracy for more on this). If an election were held tomorrow, it would almost certainly push us over ENP 4. In 2010 it may well happen anyway.
Amused Cynicism 
Doesn’t PR risk making tiny extremist parties into kingmakers which hold the balance of power in Parliament?
It’s a possibility, but not I think a very likely one. If parliament has c. 600 MPs than a small party with 10 MPs of so is unlikely to hold the balance of power (and is unlikely to hold any power if all the larger parties are against it, which they are likely to be if it is extremist.)
What’s more vlikely to happen is the situation in Scotland, where one or two of the large parties are in government, possibly a minority government.
Only if we decide to switch to stupid systems like is used in Italy and Israel, but no one is proposing that.
This doesn’t happen in Ireland or Malta, where STV is used, and certainly doesn’t happen in Germany or Spain.
Actually not true—go look up the results of the 1950 and 1951 GEs on Wikipedia (accurate according to my references).
The Labour party put up almost a full slate of candidates, the Conservatives did not—they had an electoral pact with the National Liberal party which was one of the two remnants of the old Liberals. The two Liberal parties between them got 12% of the national vote and 25 seats, but because The NLs had agreed to support Churchill it all got a bit weird. Especially in ’51.
Where Labour got a lot more votes but lost the # of seats. But then, they’d only just gerrymandered the system to create single member seats everywhere, it was that that killed off the 3rd parties completely, not just FPTP. It’s single member FPTP that really favours a 2 party system.
Elections are an iterated series, game theory has a lot to say about how they’ll end up, Duverger was right. Labour introduced SM-FPTP to their advantage as they thought, but then the Tories figured out how to abuse it better. 50 years later…
Wait, 50/51 are 60 years ago. Carry on, you’re completely correct, it takes about ten years for a new system to change the party structure, I’ll shut up.
FPTP implies single-member consituencies. If you have simple elections in multi-member constituencies you have Block Voting (if you have as many votes as there are constituency members) or Single Non-Transferable Vote (if you have only one vote).