Opinion polling for the 2010 United Kingdom general election

Since each MP is elected separately by the first past the post voting system, it is impossible to precisely project a clear election outcome from overall national shares of the vote. Not only can individual constituencies vary markedly from overall voting trends, but individual countries and regions within the nation may have a very different electoral contest that is not properly reflected in overall share of the vote figures.

Therefore, the first past the post system means that the number of MPs elected may not reflect the overall popular vote share across the parties. Thus, it is not necessarily the party with the largest share of the popular vote that ends up with the largest number of MPs. (See details of the elections in 1951 and February 1974) Since 1935 no party has achieved more than 50% of the popular vote in a British general election. The voting system favours parties with relatively concentrated support: a widely distributed vote leaves a party at risk of getting a large vote share but doing poorly in terms of numbers of seats (as the SDP–Liberal Alliance did in the 1980s), whereas parties with localised votes can win seats with a relatively small share of the vote.[4]

That said, in previous elections, approximate forecasting of results were achieved by assuming that the swing in each individual constituency will be the same across the country. This system, known as uniform national swing (UNS) is used by much of the media in Britain to assess and extrapolate electoral fortunes from opinion poll data, though there has been criticism that such predictions may be naive and unreliable, even from providers of such data.[5] By using UNS projections, several media commentators and politicians have suggested that significant swings towards the Liberal Democrats in the opinion polls may not necessarily amount to significant gains in terms of parliamentary seats, including predictions that even if the Liberal Democrats had the most votes, and Labour the least, it could be the case that Labour retains the most seats while the Lib Dems have the fewest.[6][7][8]

Normally governments can easily survive for a full parliamentary term on a majority of more than 20 seats over all other parties. Below that level there is a danger of by-elections and MPs crossing the floor of the House of Commons reducing the government to a minority such that it would be at increased risk of losing a vote of no confidence.

Immediately following the previous general election, the Labour Party held a double-digit lead in opinion polls. However, over the course of 2005, this lead was eroded somewhat. By December, the Conservative Party showed its first small leads in opinion polls following the controversial 90 days detention proposals and the election of David Cameron as Conservative leader.[9]

In early 2006, opinion polls were increasingly mixed with small leads given alternately to Labour and Conservative. From the May 2006 local elections, in which Labour suffered significant losses, the Conservatives took a small single-digit lead in opinion polls. This was the first consistent lead of the opinion polls that the Conservatives had enjoyed for 14 years.

Labour regained the lead in June 2007, following the resignation of Tony Blair as prime minister and the selection of Gordon Brown as his successor. Brown resisted calls from his party to hold a general election, despite opinion polls suggesting that Labour was capable of being re-elected at this stage. From November 2007, however, the Conservatives again took the lead and, from then, extended their lead into double digits, particularly in response to the MPs' expenses scandal and the economic recession along with the increased unemployment that resulted from it, although there was some evidence that the lead narrowed slightly towards the end of 2008 and again in late 2009. By the end of February 2010, Ipsos MORI, ICM, YouGov and ComRes polls had all found a sufficient narrowing of the Conservative lead for media speculation about a hung parliament to return - scenario which could have allowed Labour to cling onto power in a minority or coalition government.[10]

From 15 April 2010, following the first televised debate of the party leaders, however, polling data changed dramatically, with the Lib Dem vote proportion rising to 28–33%, and the Conservative vote proportion falling. In some polls, the Liberal Democrats took the lead from the Conservatives by a narrow margin which was unprecedented in the period since the Lib Dems were founded in 1988. Under UNS projections, this made a hung parliament highly probable, if Lib Dem performance had persisted.[11]

Following the second debate on 22 April the polls, on average, placed the Conservatives in the lead on 33%, the Liberal Democrats in second on 30% and Labour in third on 28%. If these polls had reflected the election day results on a uniform swing nationwide, Labour would have had the most seats in a hung Parliament, and therefore it still appeared possible that Labour might remain in power as the main party in a minority or coalition government.

At 10 pm on election day, coinciding with the closure of the polls, the results of an exit poll collected for the BBC, Sky and ITV news services were announced. Data were gathered from individuals at 130 polling stations around the country. The results of the poll initially suggested a hung parliament with the Conservative Party 19 seats from a controlling majority; this was later adjusted to 21 seats. The distribution of seats amongst the Conservatives, Labour, Liberal Democrats and other parties was initially suggested to be 307, 255, 59 and 29 respectively,[12] although the seat numbers were later changed to 303, 251, 69, and 27 respectively.[13]

Initial reaction to the exit poll by various commentators was of surprise at the apparent poor prospects for the Liberal Democrats[14] because it was odds with many opinion polls undertaken in the previous weeks. However, the actual results showed that the exit poll was a good predictor.

A later BBC Exit poll (05:36 BST) predicted the Conservatives on 306, 20 short of an overall majority, Labour on 262, and Liberal Democrats on 55.[13]

  Conservatives

  Labour

  Liberal Democrats

  UKIP

  BNP

  SNP

  Green

  Respect

The following graph shows YouGov poll results since the calling of the general election.

The following graph shows ComRes poll results recorded over the period 11 April – 6 May 2010, including annotations of the three TV debates.

Poll results are initially listed in reverse chronological order showing the most recent first, using the date the fieldwork was done, as opposed to the date of publication.

The figure given in the 'lead' column before the televised leaders' debates is the lead held by Labour or the Conservatives over the second placed of the two parties. For figures after the first debate, after which the Liberal Democrats were placed in first or second position in some polls, the second placed party is also noted in the column where applicable.

Most of the polling companies listed are members of the British Polling Council, and abide by its disclosure rules. BPIX is not a member of the BPC, and does not publish detailed methodology and findings.