Backgrounder: British general election in 2015

Britons went to the polls on Thursday in the most uncertain general election for decades and are likely to deliver a clear victory to no party.

Britain's first-past-the-post voting system delivered majority governments in all seven general elections from 1979 to 2005.

But the old certainties of the nation's politics - based on two main parties split between left and right and with a strong class foundation - have faded to the extent that the 2010 election saw no party gain overall control, and Conservative leader David Cameron had to create a coalition with the centrist Liberal Democrats to become prime minister.

In this election that support has fallen further, with both main parties on about 30 percent to 35 percent of the voters, while smaller parties have gained in popularity.

The UK Independence Party (UKIP), which campaigns on an anti-European Union (EU) and anti-immigration agenda, has risen on the right. Pollsters predict it may get 12 percent of the vote at the election.

Because of the winner-takes-all nature of the British constituency system, UKIP may see this support translate into just one or two seats in parliament, but its success has led the main parties to move to the right on immigration.

Other small parties have benefited from the decline in support for the three main parties.

On the left, the Greens now pick up 7 percent of the vote, and the nationalist parties of Wales and Scotland - Plaid Cymru and the Scottish National Party (SNP) - have increased support.

This is especially true of the SNP which, despite the defeat in a referendum on Scottish independence last year, is building on the support it won then of 45 percent of Scots voters.

It now looks likely to take most of the parliamentary seats in Scotland, seizing many off Labour which has traditionally dominated the Scottish parliamentary vote.

The SNP is left-wing, and has vowed to keep the Conservatives out of power. Its haul of seats, possible up to 46 or even more, could help to oust Cameron from Downing Street and replace him with Labour's Ed Miliband.

Whoever forms the next government is likely to rely on the formal support of other parties in a coalition - like that of 2010-15 - or to govern as a minority party.

The next government will have to deal with the issues of continued pressure on government spending, membership of the EU (with the Conservatives promising a referendum in 2017) and strong nationalist feeling in Scotland.

The latter two pose questions about the long-term future of Britain, and they set a tough challenge for whichever leader becomes prime minister. Endit