General elections are held after Parliament has been ‘dissolved’, either by a royal proclamation or because the maximum term between elections - five years - has expired. The decision on when to hold a general election is made by the Prime Minister.
For electoral purposes Britain is divided into constituencies, each of which returns one MP to the House of Commons. MPs are elected by the relative majority method - sometimes called the ‘first past the post’ principle - which means the candidate with more votes than any other is elected.
In elections to the Scottish Parliament, the National Assembly for Wales and the European Parliament in 1999, forms of proportional representation (PR) were used for the first time in Great Britain. PR was also used in the 1998 elections for the Northern Ireland Assembly. Northern Ireland has used a version of PR in European Parliament elections since 1979.
All British citizens together with citizens of other Commonwealth countries and citizens of the Irish Republic resident in Britain may vote, provided they are aged 18 years or over and not legally barred from voting. People not entitled to vote include those serving prison sentences, peers and peeresses who are members of the House of Lords, and those kept in hospital under mental health legislation. Voting is by secret ballot. At a general election the elector selects just one candidate on the ballot paper and marks an 'X' by the candidate's name. Voting in elections is voluntary. In the June 2001 general election 59.4 per cent of the electorate voted, compared with 72 per cent in 1997.
Any person aged 21 or over who is a British citizen or citizen of another Commonwealth country or the Irish Republic may stand for election to Parliament, provided they are not disqualified. People disqualified include those who are bankrupt, those sentenced to more than one year’s imprisonment, members of the clergy, members of the House of Lords, and a range of public servants and officials. Approved candidates are usually selected by their political party organisations in the constituency which they represent, although candidates do not have to have party backing.
The leader of the political party which wins most seats (although not necessarily most votes) at a general election, or who has the support of a majority of members in the House of Commons, is by convention invited by the Sovereign to form the new government.

Main source: UK Government Organisations in Japan