Campaigning is underway in Britain for the general election on May 7 that is expected to be one of the country's closest in decades.

Here's what you need to know about the political parties and possible scenarios:

Key Parties

1. Conservative Party

The centre-right Conservatives, led by Prime Minister David Cameron, have been senior partners in the coalition government with the centrist Liberal Democrats since 2010, but are battling to win enough seats in the House of Commons to govern alone this time. They are hoping to capitalise on a strong economic recovery to win the election. Mr Cameron has said if he wins the polls, he will renegotiate the terms of Britain's European Union (EU) membership ahead of an in-out referendum to be held by the end of 2017. He has also promised to reduce annual net migration from EU nations to the "tens of thousands" from around 300,000 currently in a bid to reduce pressure on wages, schools and medical services.

While many companies want to see EU reforms, the prospect that Britain could leave the bloc, a scenario dubbed "Brexit", has unsettled those fearful of losing their access to Europe's single market of 500 million people.

The Conservatives are in a neck-and-neck race with the Labour Party in most opinion polls.

2. Labour Party

Mr Ed Miliband's Labour Party has traditionally been the biggest party in Scotland but it is struggling to stem what looks like a devastating loss of support to the Scottish nationalists who have surged despite losing a referendum on independence last year. That makes it harder for Labour to win outright.

The party has centred its campaign on the treasured but expensive state-funded National Health Service (NHS) which provides mostly free, across-the-board healthcare. Mr Miliband has pledged to protect the NHS from what he claims is creeping privatisation under Mr Cameron's coalition. He has promised a new five-per-cent cap on profits for private companies which take on NHS contracts worth over £500,000 (S$1.02 million) and vowed to spend £2.5 billion more than the Conservatives on the health service.

3. Liberal Democrats

The Liberal Democrats have seen their support plunge over the past five years. But they hope to convince past supporters that they deserve another chance based on their record as part of the coalition.

Some believe leader Nick Clegg would strike a second coalition deal with the Conservatives after the election, backing legislation to allow an in-out EU referendum in return for major concessions. But in a sign of the divisions with the party, senior member and Energy Secretary Ed Davey has said it would be "incredibly difficult" to govern with the Conservatives again because of their position on Europe and environmental policy. He said the country had made huge progress in the last five years on green issues and is now a respected international player, but a government that is prepared to entertain the idea of leaving the EU would lose influence in climate change negotiations, including crucial talks in Paris at the end of this year.

4. United Kingdom Independence Party (UKIP)

The Conservatives are challenged by UKIP, a populist movement headed by Mr Nigel Farage who also wants to take the country out of the EU and impose strict immigration controls against all foreigners. UKIP would be lucky to win more than a handful of seats, but it could eat into the Conservatives' supporter base in south England where it is now polling best.

Britain's first-past-the-post electoral system means UKIP is unlikely to win more than half a dozen of 650 parliamentary seats while finishing second in large numbers of them. But it has the power to take large numbers of Conservative votes, and some from Labour too, making it harder for Mr Cameron to win. In a three-cornered fight, UKIP could deliver many constituencies that should remain in the Conservatives' hands to Labour.

5. Scottish National Party (SNP)

Led by Ms Nicola Sturgeon, current First Minister of Scotland, the SNP vows to gain Scotland's independence from Britain. In theory, the SNP's appeal should have faded after a referendum last September decisively rejected Scotland's separation. But the SNP has skilfully played on the Scots' disenchantment with the central government in London and now looks set to gain up to 50 of Scotland's 75 parliamentary seats, all at the expense of Labour, which used to treat Scotland as its fiefdom.

6. Green Party

The Greens are led by Ms Natalie Bennett, an Australia-born former journalist. They are polling in the 5-10 per cent band, regularly beating the Liberal Democrats, and claim that membership has quadrupled in the past year, overtaking the Liberal Democrats and UKIP. Having outflanked the most left-wing Labour Party in decades by pledging to tear up austerity, halt the Transatlantic Trade and Investment Partnership (TTIP) global trade deal and renationalise the railway network, the Greens believe they can grow from one seat to three and finish second in a dozen more constituencies.

With polls suggesting a hung parliament could be likely after the election, the party had previously floated the idea of a "progressive alliance" also involving the SNP. Ms Bennett said the Greens would not support a Conservative minority government, but would consider a "vote by vote" arrangement with Labour.

7. Plaid Cymru

Ms Leanne Wood's party, which currently holds three parliamentary seats, has said it would work with the SNP and the Greens. The national party in Wales has vowed to use its influence in a hung parliament to "unleash Wales' economic potential". It is calling for extra powers and resources for Wales, support for "permanently rebalancing the UK economy" and an end to austerity.

Four possible outcomes

1. Majority government

One party wins a clear majority of at least 326 of 650 seats. But pundits do not think this is likely.

2. Coalition government

This outcome would deliver a stable government even though no single party has a clear majority. It would see either the Conservatives or Labour make a formal deal with a smaller party such as the Liberal Democrats to govern. The Conservatives have been in a coalition government with the Liberals since 2010. But as this arrangement has created problems for both, it is seen as less likely this time around.

3. Minority government

If no party wins a clear majority and a coalition government is not formed, either the Conservatives or Labour could attempt to form a minority government. But this could prove unstable as the government would have to depend on support from smaller parties on key issues. The UKIP could prop up a Conservative minority government, while the SNP is pushing for such an arrangement with Labour.

The first big test of confidence in a minority government would be the Queen's speech a few weeks after the election, when Parliament debates the new government's legislative programme and then votes on it.

4. Fresh elections

A minority government could struggle to sustain support for long. One option for the prime minister leading it would be to hold a new election quickly in a fresh bid to secure a clear majority. This could mean Britain holds two general elections in a year for the first time since 1974.
(The Straits Times)