There has been speculation that the UK could end up with a hung Parliament - one where no single party has a majority - after the general election. Justin Parkinson looks at the five times it has happened in the last 100 (and a bit) years.
JANUARY AND DECEMBER 1910
Asquith's Liberal government received backing from Irish nationalist MPs
The Liberals had governed since 1906 with a Commons majority, although they had also been backed by Labour, which supported social reform, and the Irish nationalists - on the understanding that Home Rule would pass into law.
The election of January 1910 followed the rejection by the House of Lords of Liberal Chancellor Lloyd George's "People's Budget", which included a "super tax" on the wealthy.
Voters returned more Liberal MPs - under leader Herbert Asquith - than those of any other party. They took 275 seats, two more than Arthur Balfour's Conservatives, but were 61 short of a Commons majority. Labour took 40 and the Irish Nationalists 82.
The Irish Nationalists kept the Liberals in power and a further election, in December 1910, gave the Tories and Liberals the same number of seats - 272. This figure was 64 short of a majority.
The nationalists, with 84 seats, again kept the Liberals in power. The government also brought in welfare and trade union legislation which was backed by Labour.
The main parties formed a coalition during World War I and, in 1916, Lloyd George joined the Conservatives in ousting Asquith, becoming prime minister himself.
At the next general election, in 1918, the coalition won a majority - in the face of opposition from splinter groups from all the main parties - and carried on with Lloyd George as prime minister, even though the Conservatives won most seats.
Stanley Baldwin was back in power a few months after standing down
Conservative Prime Minister Stanley Baldwin was in for a tumultuous, but ultimately victorious, 10 months when his party failed to win a majority in the election, held on 5 December 1923. His was still the largest single party, with 258 seats - but 50 short of a majority. Meanwhile Labour took 191 and the Liberals 158 - ceasing to be the UK's second party.
On 24 January 1924, six weeks after the election, Baldwin went to the Commons, still as prime minister, to ask MPs if he could continue. But he lost a confidence vote and resigned. A minority Labour administration - the first Labour government - was formed under Ramsay MacDonald, with the support of, but not a pact with, the Liberals.
Over the summer of 1924, the government decided not to prosecute JR Campbell, editor of the communist Workers' Weekly, for sedition, after the publication of an article urging the services not to fight in the event of a war.
The government made the decision a no-confidence issue, and was defeated by the opposition parties. It called another election.
Baldwin was helped by the so-called Zinoviev letter - published in the Daily Mail four days before polling. It purported to show that an international communist conspiracy existed to stir up agitation in the UK, including among the armed forces.
At the 24 October election, many Liberal voters, furious that their party had helped "socialists" into power, deserted the cause. The Liberals were routed, dropping to just 40 seats, and the Tories came back to power with a landslide.
MacDonald reportedly told his cabinet, just before leaving office, that he felt "like a man sewn in a sack and thrown into the sea".
MacDonald's decision to join a coalition split the Labour Party
This time round, on 30 May 1929, Labour won the most seats - 287 - for the first time. But it was still 21 short of a Commons majority.
Conservative Prime Minister Stanley Baldwin resigned, refusing to negotiate with Liberal leader David Lloyd George, whom he detested, regarding staying in office under such circumstances as humiliating.
So, a second minority Labour government was formed, with Ramsay MacDonald again becoming prime minister.
During the next couple of years, the economy deteriorated, with ministers unable to arrest the slump.
In 1931, sterling seemed in great danger, and King George V called a Buckingham Palace conference to discuss plans for a government of "national unity". This took over on 31 August, promising to balance the Budget.
MacDonald stayed as prime minister, although all but 14 Labour MPs refused to come across to the national unity government with him. The Liberals also split into pro- and anti-national government factions.
MacDonald called an election for 24 October.
The Conservatives won a massive 470 seats. MacDonald's National Labour - which did not compete with the Tories - took 41. Labour, led by Arthur Henderson, took just 52.
So, the national unity government persisted, with a huge majority. MacDonald - still regarded as a traitor by many in the Labour Party today - stayed on as prime minister until 1935, when Baldwin took over.
Wilson went to the country again before the year was over
Conservative Prime Minister Edward Heath, embroiled in conflicts with the unions, called an election for 28 February 1974. Labour became the biggest party - with 301 seats - but was 17 short of a Commons majority. The Conservatives took 297 seats. The Liberals won 14, meaning that they did not hold the balance of power.
Heath stayed in Downing Street for four days, until he realised he could not put a coalition together. He had offered this to Liberal leader Jeremy Thorpe, who called in return for the Tories to bring in a proportional representation voting system. Heath was unable to make this promise.
Heath also offered the Conservative whip to seven Ulster Unionist members of the United Ulster Council - made up of the different unionist parties in Northern Ireland - which had 11 MPs in total. But the Council said the Tory whip would have to be extended to all 11 of its members. Heath was unable to agree to this, as it would mean giving the whip to Democratic Unionist Ian Paisley, who objected to the power-sharing executive for Northern Ireland, which had been established in January 1974.
Heath resigned and Harold Wilson's Labour formed a minority government, which lasted until October, when there was another election. This time Labour won a majority of three seats in the Commons. However, by April 1976 it had become a minority government because of by-elections and defections.
Amid troubling economic times, Labour formed a pact with the Liberals from March 1977 to July 1978. This worked on the basis that the Liberals would support Labour in any no-confidence vote.
After the pact ended and the "winter of discontent" of 1978/9 - when strikes brought much of the UK to a halt - Labour lost a confidence vote, called an election and was beaten by Margaret Thatcher's Conservatives.