LONDON (Reuters) - David Cameron can thank Britain’s winner-takes-all voting system for handing him an outright majority in parliament on just 37 percent of the vote. But by boosting the power of Scottish nationalists, it has also intensified one of his biggest headaches.

Britain's Prime Minister David Cameron and his wife Samantha arrive at Westminster Abbey for a thanksgiving service on the final day of 70th anniversary Victory in Europe (VE) day commemorations in central London May 10, 2015. REUTERS/Suzanne Plunkett

The results of Thursday’s election highlight more starkly than ever the anomalies of a system that allocates seats not according to the parties’ total number of nationwide votes but on the basis of 650 local ‘first-past-the-post’ contests.

Conservative leader Cameron earned a second term as prime minister with 11.3 million votes and 331 of the 650 seats. But only one of those seats was in Scotland, where the pro-independence Scottish National Party (SNP) won 1.45 million votes, half of those cast, and took 56 of the 59 Scottish seats.

The resulting electoral map portrays a starkly divided Britain.

In England, it is dominated by Conservative blue except for the red strongholds of the opposition Labour party in London and the big cities of the north and midlands. In Scotland, it turns bright nationalist yellow, an outcome that former SNP leader Alex Salmond called a ‘staging post’ on the way to independence.

Those complaining loudest are the anti-European Union UK Independence Party and the Greens. Nationwide, they took 12.6 percent and 3.8 percent of the vote respectively, but their support was too thinly spread to win more than one member of parliament each.

“The fact that over 5 million people between them have voted UKIP and Green, and they have two MPs, strikes us as utterly absurd and a tragic denial of people’s democratic wishes,” said Will Brett, head of campaigns for the Electoral Reform Society which advocates a switch to proportional representation (PR).

By Sunday morning, 100,000 people had signed a petition launched by the society and another campaign group, Unlock Democracy, that states: “The 2015 general election has shown once and for all that our voting system is broken beyond repair.” It urges politicians of all parties to embrace reform.

LONG BATTLE

Complaints about the fairness of the system are not new. For decades it was the centrist Liberals, and their successors the Liberal Democrats (Lib Dems), who led the calls for change.

As part of the price for supporting Cameron in a coalition after the previous 2010 election, the Lib Dems were granted a referendum in 2011 on adopting a modified version of first-past-the-post (FPP), in which voters would rank candidates in order. The change was rejected, on a low voter turnout.

Advocates of FPP say it is a tried and tested system that for the most part has delivered clear election outcomes and stable governments. But opponents say the Scottish question, the fragmentation of the old two-party-dominated political structure and the emergence of movements such as UKIP and the Greens have all bolstered the case for reform.

Under a system of PR like the one used in Germany and New Zealand, Cameron’s Conservatives would still have been the biggest party in the House of Commons after Thursday’s election but would have needed to rely on UKIP, with more than 80 seats, to scrape a majority.

In Scotland, the SNP would be reduced to half the seats, with the others split between Labour, the Conservatives and Liberal Democrats. In that more balanced landscape, the political chasm between England and Scotland would be greatly narrowed.

Even before the election, that point was forcefully argued by Vernon Bogdanor, a constitutional expert who was Cameron’s politics tutor at Oxford University.

“Distorted representation makes the UK appear more divided than in fact it is,” he wrote in Prospect magazine in February. “Proportional representation, therefore, would alter the dynamics of the conflict between England and Scotland and make it far more manageable.”

No one expects Cameron to change the system that has swept him back to power. But in the long term, some argue, there are compelling reasons to re-draw electoral rules that divide much of Britain into fortresses held for decades by one party or the other.

That means that millions of people are effectively disenfranchised and elections are decided in a relatively small number of close-fought marginal seats.

While the current system gives Cameron a mandate for a majority government, “it’s not such a strong mandate that he can ignore the rest of the country,” Brett said.

“It’s going to be hard to ignore electoral reform indefinitely.”