The UK Supreme Court begins two days of hearings on Tuesday to consider whether the Scottish government can pass legislation allowing it to hold a second independence referendum without the approval of the British government.
In a referendum in 2014, Scots voted 55%-45% to remain in the United Kingdom, but nationalists argue that the vote for Brexit two year later changed everything.
The Scottish government has said it wanted to hold a second vote next year.
Below is the history of the push for independence and how another vote could happen:
ACT OF UNION
The nations of Britain have shared the same monarch since 1603, when King James VI of Scotland became James I of England. In 1707, a formal union created the Kingdom of Great Britain.
Now, the United Kingdom of Great Britain and Northern Ireland binds England, Scotland, Wales and Northern Ireland and has an overall population of about 68 million, of which Scots make up some 5.5 million.
In 1998, the then Labour government passed the Scotland Act which created the Scottish parliament and devolved some powers from Westminster.
ONCE-IN-A-GENERATION VOTE
Both sides agreed at the time of the 2014 plebiscite that it should be a once-in-a-generation poll. However, nationalists say Brexit was a game changer. While the United Kingdom as a whole voted in favour of leaving the European Union in 2016, a clear majority in Scotland voted to stay in the bloc.
Independence supporters say one of the main arguments put forward in 2014 by opponents of a break-up was that an independent Scotland could not join the EU.
The left-wing, nationalist Scottish National Party (SNP), which has run Scotland's devolved assembly since 2007, also argues that the UK government has pursued policies with which the vast majority of Scots disagree.
In the last national election for the UK parliament in 2019, the SNP won 45% of votes cast and 48 of the 59 Scottish seats, while Britain's ruling right-wing Conservative Party captured just six.
ELECTION RESULT OF 2021
In elections for the Scottish parliament in May 2021, the SNP said it would seek to hold a referendum if pro-independence parties won a majority.
The SNP together with the Scottish Greens, who also support secession, won more than half of 129 seats in the parliament, giving a pro-independence majority to ensure any referendum bill could be passed.
In June this year, Scottish First Minister and SNP leader Nicola Sturgeon announced plans to hold an independence referendum on Oct. 19, 2023.
Opinion polls since the 2014 vote have mostly continued to show a majority of Scots support remaining in the UK. A YouGov poll conducted between Sept. 30 and Oct. 4 found 45% would back independence, with 55% saying they would vote no.
THE LEGAL BARRIER
Under the Scotland Act, all matters relating to the "Union of the Kingdoms of Scotland and England" are reserved to the UK parliament in London.
Westminster can grant the Scottish government the authority to hold a referendum using a so-called "Section 30" order, a process that was used to allow the 2014 plebiscite to go ahead.
But Prime Minister Liz Truss and her predecessor, Boris Johnson, have repeatedly said the 2014 vote should not be repeated for a generation and refused permission for a referendum.
Sturgeon argues this is blocking the democratically expressed will of the Scottish people, and that a referendum should be held regardless. However, she says for any referendum to be effective, it must be lawful and recognised as such by the international community.
She has cited the case of Catalonia which unsuccessfully declared independence from Spain in 2017 after a referendum ruled illegal by judges.
In order to establish the legal position, the most senior law officer in Scotland has referred the question of whether the Scottish government can pass a referendum bill to the Supreme Court, the United Kingdom's top judicial body.
LIKELY OUTCOME
While Britain has an unwritten constitution, meaning much of it can be subject to interpretation, the Scotland Act lays out the relationship between the Scottish assembly and the UK parliament.
The British government argues that it is clear, and that all matters do with the union of the nations are reserved to the Westminster parliament.
The SNP says the right to self-determination is "fundamental and inalienable", and there was no practical means to push for a referendum through the UK legislature.
Legal experts say it will be difficult for the Scottish government to convince the five Supreme Court judges hearing the case that a referendum bill would be within the legislative competence of Scotland's parliament.
NOT THE END
Sturgeon has promised that defeat in the Supreme Court would mean the SNP would fight the next UK-wide election, due to be held in 2024, solely on a platform of whether Scotland should be independent, making it a "de facto" referendum.
However, that stance would also likely provoke further legal challenges.
If there were a referendum and Scots voted to leave, it would be the biggest shock to the UK since Irish independence a century ago - just as the UK grapples with the energy crisis, the ongoing impact of the COVID-19 pandemic and the consequences of Brexit.
An independent Scotland would also have to address issues such as what currency to use, and how long the Bank of England would determine monetary policy and set Scottish interest rates.
(Reporting by Michael Holden; Editing by Nick Macfie)
