<P> Most candidates were representatives of a political party registered with the Electoral Commission . Candidates not belonging to a registered party could use an "independent" label, or no label at all . </P> <P> The leader of the party commanding a majority of support in the House of Commons is the person who is called on by the monarch to form a government as Prime Minister, while the leader of the largest party not in government becomes the Leader of the Opposition . Other parties also form shadow ministerial teams . The leaders of the SNP, Plaid Cymru and the DUP are not MPs; hence, they appoint separate leaders in the House of Commons . </P> <P> The Conservative Party and the Labour Party have been the two biggest parties since 1922, and have supplied all Prime Ministers since 1935 . Both parties changed their leader after the 2015 election . David Cameron, who had been the leader of the Conservative Party since 2005 and Prime Minister since 2010, was replaced in July 2016 by Theresa May following the referendum on the United Kingdom's membership of the European Union . Jeremy Corbyn replaced Ed Miliband as Leader of the Labour Party and Leader of the Opposition in September 2015, and was re-elected leader in September 2016 . </P> <P> While the Liberal Democrats and their predecessors had long been the third - largest party in British politics, they returned only 8 MPs in 2015--49 fewer than at the previous election . Tim Farron became the Liberal Democrat leader in July 2015, following the resignation of Nick Clegg . Led by First Minister of Scotland Nicola Sturgeon, the SNP stands only in Scotland; it won 56 of 59 Scottish seats in 2015 . UKIP, then led by Nigel Farage, who was later replaced by Diane James and then by Paul Nuttall in 2016, won 12.7% of the vote in 2015 but gained only one MP, Douglas Carswell, who left the party in March 2017 to sit as an independent . After securing 3.8% of the vote and one MP in the previous general election, Green Party leader Natalie Bennett was succeeded by joint leaders Caroline Lucas and Jonathan Bartley in September 2016 . Smaller parties that contested the 2015 election and chose not to stand candidates in 2017 included Mebyon Kernow, the Communist Party of Britain, the Scottish Socialist Party and the National Front . </P>

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