There are no fixed rules in the UK for when a referendum should take place.
Referendums are held on an ad hoc basis, as and when Parliament decides. There have only ever been three UK-wide referendums: in 1975 and 2016 on EEC/EU membership, and in 2011 on electoral reform. There have been many more referendums on devolution to particular parts of the country.
Referendums are generally either 'confirmatory', when the public has the chance to approve or reject a proposition of the Government; or 'advisory', when the public can advise the Government on what policy to implement. The 2011 referendum on electoral reform was confirmatory, with a binary choice between two clearly defined voting systems. The 2016 EU referendum, on the other hand, is generally regarded as advisory.
A further referendum on Brexit would only happen with the Government’s support. It would require Parliament to pass primary legislation, to set the date for the referendum and how long each aspect of the referendum process will take.
The UK stands in contrast to countries such as Ireland, where a referendum must be held to approve any constitutional amendment, and Switzerland, where referendums take place if a specified proportion of voters sign a petition in favour.
Most referendums offer voters a binary choice between two options. In the case of Brexit, there are at least three possible outcomes: Theresa May’s deal, Brexit with no deal, or remaining in the EU after all.
A second referendum could in principle be held as a straight choice between two of these options: for instance Deal vs No Deal, or Deal vs Remain. In this case, whichever option won more votes would be implemented. This approach would naturally be opposed by those who support the third, excluded option.
If a referendum were held that includes all three options then there are different ways to determine the result. The simplest approach would be to allow each voter to vote for their favoured option, with the most popular choice declared the victor. This could lead, however, to an option being implemented without support of a majority of voters.
To avoid this, voters could rank their choices in order of preference. The least popular first choice would be eliminated, and its votes reallocated based on second preferences. This would ensure that the successful option gained over 50% of first and second preferences.
Finally, the referendum could include two separate questions. Question 1 could ask whether voters wish to Leave or Remain, as in 2016. Question 2 could ask voters to select between the two different models of Brexit, if there is still a pro-Leave majority.
Parliament would specify the question to be put to the people in any legislation authorising a second referendum. However, under the terms of the Political Parties, Elections and Referendums Act 2000 (PPERA) the Electoral Commission has a statutory role to play in assessing a referendum question, to ensure it is intelligible and free from bias.
In 2015, the Government initially proposed that the EU referendum question be “Should the United Kingdom remain a member of the European Union?”. The Electoral Commission concluded that this question might be biased in favour of the ‘Yes’ option. Its recommendation, accepted by Government, was that this should be replaced by the familiar choice between Remain and Leave.
Past practice provides a guide to the stages for a further referendum. The first step would be the drafting and introduction of legislation to enable a referendum to take place. The bill would then need to pass through Parliament: this took seven months in the case of the 2016 referendum, but the process would have to happen much more quickly in current circumstances.
The Electoral Commission normally takes up to 12 weeks to test the proposed question but this could also be curtailed. The Constitution Unit thinks this task could be completed in a minimum of eight weeks, and that Parliament could complete the legislative process in a further three weeks if necessary. The length of the referendum campaign would also be specified in legislation. The formal, regulated referendum period must last a minimum of 10 weeks, under the Political Parties, Elections and Referendums Act (PPERA).
A fair estimate is that the whole process would take a minimum of 21 weeks, but this would be much shorter than other recent referendums.
The Electoral Commission also has the responsibility for nominating lead campaign groups for each possible referendum outcome. In a three-way referendum there would presumably be three official campaigns.
The official campaigns gain benefits including direct government funding, free campaign broadcasts and free postage of campaign materials.
In 2016 the Prime Minister relaxed collective responsibility to allow ministers to campaign on both sides of the EU referendum, even though government policy was in favour of Remain. In the event of a second referendum, the Prime Minister would likewise have the choice of whether to allow ministers to campaign against the official government position.
Legislation for a second referendum would specify who can vote (the electoral franchise). In the 2016 EU referendum, the UK parliamentary franchise was used. This meant UK, Irish and Commonwealth citizens over 18 years of age and resident in the UK were entitled to vote, along with UK citizens living abroad for less than 15 years.
There are currently about 46 million registered voters for UK parliamentary elections, and an estimated 6 million people who are eligible but not registered.
The default assumption is that the same franchise would be used in any second referendum, although that would be contested and could be amended by Parliament.
EU citizens, under-18s and other UK expats are currently ineligible to vote. There are an estimated 2.6 million EU citizen adults in the UK, 3 million ineligible UK expats, and 1.5 million 16- and 17-year-olds living in the UK.