Why a referendum?
Prime Minister David Cameron -under pressure both from within his Conservative Party and from outside, especially by the rise of the United Kingdom Independence Party, UKIP- committed over the last election campaign to hold a referendum on UK's EU membership. Cameron on February 20th announced the date of the referendum -June 23rd- after reaching an agreement on a new, special status for the UK within the EU. According to the deal, the UK "is not committed to further political integration into the European Union." The agreement accepts that the UK introduces 7-year limitations to welfare benefits for non-UK workers.
Stances by UK-wide parties
The Labour Party, the Liberal Democrats and the Green Party officially support the UK remaining in the EU. The Conservative Party is divided over the issue. Cameron favours the "remain" option after the deal he struck with the EU, but several government ministers will campaign for the "leave" option. UKIP will obviously be campaigning for a vote to leave.
The "remain" official campaign: Stronger In
The Britain Stronger in Europe group (Stronger In, in short) has been declared the official "remain" campaign by the Election Commission. The group emphasizes the economic prosperity and security which, according to its supporters, the United Kingdom enjoys being an EU member. Most of its political support comes mainly from the Labour, the Conservatives and the LibDems. Conservative peer Lord Rose and Labour member Will Straw are the campaign's chairman and executive director. Cameron supports Stronger In, although he has no official role in it. The BBC lists supermarket magnate Lord Sainsbury and investment banks Goldman Sachs and Citi among Stronger In's main financial backers.
Other, alternative "remain" campaigns do exist. One of them is Another Europe is Possible, which highlights that the EU safeguards social rights. The group is mainly backed by left-wing supporters.
The "leave" official campaign: Vote Leave
On the other side, Vote Leave has been declared the official "leave" campaign, with most of its support hailing from the Conservative Party and UKIP, although some prominent Labour members have also joined it. Vote Leave argues the UK should no longer be subject to EU institutions, which the group believes to be "wrong" and "anti-democratic". The group also says the UK should regain control over migration flows. One of the most outstanding leaders who will be campaigning for Vote Leave is London mayor Boris Johnson. The campaign is funded, among others, by multi-millionaires such as Conservative donor Peter Cruddas, Labour financial backer John Mills, and UKIP donor Stuart Wheeler. Vote Leave is headed by political campaigner and Eurosceptic group Business for Britain founder Matthew Elliott. Dominic Cummings, former advisor to the UK conservative government, is the group's campaign director.
As with the "remain" campaign, other alternative campaigns exist, such as Grassroots Out, Leave.eu, and the Trade Union and Socialist Coalition (TUSC), which argues the EU acts against the interests of the working class.
Scotland, Wales, Northern Ireland and Cornwall
Scottish, Welsh, Northern Irish and Cornish main pro-independence or federalist parties are all against leaving the EU. This includes the Scottish National Party (SNP), the Scottish Socialist Party (SSP) -in this case as a "lesser evil"-, Plaid Cymru, Sinn Féin, the Social Democratic and Labour Party (SDLP) and Mebyon Kernow.
They hold a range of arguments. Sinn Féin fears Brexit would further tear apart Ireland, leaving one side -the Republic- within the EU and the other -Northern Ireland- outside. The SNP highlights how important "social and employment protections" and "investment and exports" are, which are supported by EU membership, according to party leader and Scottish first minister Nicola Sturgeon. Plaid Cymru and Mebyon Kernow emphasize that both Wales and Cornwall have benefited from EU membership, and argue it is better to work for a more democratic and decentralized union from within.
In the event of Brexit, Sinn Féin warns that a referendum on Irish reunification should be held in Northern Ireland. The SNP too states that Brexit should immediately lead to a second referendum on Scottish independence. By contrast, the leader of the Scottish branch of Vote Leave, Labour member Tom Harris, believes Brexit would open the door to more resources for the UK and, thus, potentially to a new deal on Scotland's enlarged self-government.