The Scottish Government requires a transfer of power from Westminster - in the form of a Section 30 order - to stage a second referendum on independence.
Nicola Sturgeon will request this by Christmas from the new Prime Minister - but it's not clear if it will be granted. An agreement was achieved to hold the 2014 vote through the Edinburgh Agreement signed between Alex Salmond and David Cameron. But Boris Johnson is ruling out such a transfer if he is returned as Prime Minister after the election. Labour leader Jeremy Corbyn has hinted that he could allow it after the 2021 Holyrood elections if the SNP win a majority - but only if Labour requires SNP support to form a Government.
Nicola Sturgeon insists that Westminster's opposition to a Section 30 order will be unsustainable if the SNP wins a majority in Scotland in the election. But if this is rejected, what other options are out there for the SNP:
This would see the Scottish Government forge ahead with a vote regardless of the legal position, perhaps insisting they have a mandate from the Scottish people if not Westminster. It would win widespread support from grassroots nationalists who are hungry for action, but this approach in Catalonia only resulted in Madrid sending in troops on the ground and the leaders of the pro-independence campaign, who organised the vote, being jailed. Although such a hardline approach met with widespread criticism, it has done nothing to further the independence movement. Nicola Sturgeon has so far ruled out such an approach, insisting it won't gain international recognition
Both Nicola Sturgeon and and her Constitution Secretary mike Russell were notably cagey about the prospect of a legal challenge when quizzed about this at the SNP conference in Aberdeen last month. After the success of the SNP Home Affairs spokeswoman Joanna Cherry in spearheading the UK Supreme Court challenge against Boris Johnson's unlawful proroguing of Parliament, the party may feel going down the legal route is a feasible option. It's not clear what the grounds for challenge would be, but a number of constitutional experts believe that Holyrood may already have the authority to hold a legal referendum on independence under the devolution settlement - and the matter would at least be worth putting to the legal test. The problem is that, the Scottish Parliament would definitely not have the authority to implement independence in the event of a yes vote under such a scenario. It may nonetheless be seen as a legitimate way to test public opinion, although the the prospect of a boycott by unionists may scupper this.
Unilateral Declaration of Independence (UDI)
Senior Nationalists, including ex-Health Secretary Alex Neil and Western Isles MP Angus MacNeil have talked up such an approach. This would see the Scottish Government open independence negotiations in the event of a majority SNP victory at a Holyrood or Westminster election.
A motion was brought to the SNP's party conference seeking a change n policy to adopt UDI as a "Plan B" if a Section 30 is not successful. but it was rejected by delegates and Nicola Sturgeon has spoken out against such an approach.
Although unlikely, the SNP could opt to live a bit dangerously to increase the pressure for independence. The Scottish Government has significant powers in a range of areas which could be utilised to make life very difficult for Westminster. For example the Greens have been agitating for tighter controls on the transportation of nuclear weapons from the Faslane naval base, with some claims that Holyrood's control of environmental laws could even effectively thwart their passage through Scotland in order to be "serviced" at Aldermaston. The current SNP Transport Secretary Michael Matheson has previously sought to introduce legislation which would criminalise ministers who prepare the way for the use of nuclear weapons. The question is whether such a confrontational approach would alienate voters from the independence cause.