In total, 78 Members of the European Parliament were elected from the United Kingdom using proportional representation.

As a consequence of the 2004 enlargement of the European Union, the number of seats allocated to the United Kingdom was fewer than in 1999.

[3] A combination of the effects of the Treaty of Nice and the 2004 enlargement of the European Union meant that the number of seats allocated to the United Kingdom for the 2004 election was reduced from the 87 MEPs allocated for the 1999 election to 78 MEPs.

[4][5] As a result of the successful challenge of Matthews v United Kingdom before the European Court of Human Rights in 1999[6] residents of Gibraltar, voted in the European Parliament election for the first time, as part of the South West England region.

Labour's decline in votes was regarded as being largely due to widespread public dissatisfaction about the Iraq War and (as with the Conservatives) the increased popularity of UK Independence Party (UKIP).

The Conservatives' share of the vote declined by 2 per cent, making it the region with the smallest swing against them.

The Conservatives managed to make gains pushing Plaid Cymru into third and whose share of the vote fell by 12 percentage points relative to 1999.

Gibraltar participated in the United Kingdom's election for the first time in 2004 as part of the South West England constituency.

Gibraltar is a British overseas territory (BOT) and therefore is under the jurisdiction and sovereignty of the United Kingdom but does not form part of it.

[22] After further disagreement with the leadership, Kilroy-Silk resigned the UKIP whip in the European Parliament on 27 October 2004.

[26] UKIP formed a new European Parliament Group, Independence/Democracy which was co-chaired by Nigel Farage and Hanne Dahl.