The next prime minister of the UK is set to inherit a range of complex legal and political issues from the outgoing Theresa May. With Brexit on the horizon, here are the five key constitutional questions the next prime minister must consider, in order to safeguard the territorial integrity of the UK.

1. Northern Ireland: what to do about the backstop?

The most immediate constitutional issue facing the next prime minister relates to the Brexit impasse in Northern Ireland. With the EU remaining consistent in its position to not renegotiate the backstop, the future prime minister faces tough decisions in finding a workable solution to the Northern Irish question.

The dilemma pits the delivery of Brexit against the integrity of the union.

Should the former take priority in the form of a hard Brexit, support will likely rise for a poll on Irish reunification. Conversely, placing precedence on the territorial integrity of the UK, a softer Brexit scenario – or revocation of Article 50 – would safeguard Northern Ireland's place in the UK but cross fundamental red lines in the Conservative Party's Brexit policy.

2. Scotland: should there be another independence referendum? 

The Scottish first minister, Nicola Sturgeon, recently committed to holding a second independence referendum before 2021, which leaves the future prime minister with two options.

He may transfer to Scotland the powers to hold a second referendum. With opinion polls showing only a modest increase in support for independence since 2014, and the absence of a clear pro-independence majority, this may prove a calculated risk. However, relying on luck in referendums has only a limited change of success – as David Cameron experienced in 2016.

The second option is for the UK prime minister to withhold permission for a second referendum – as Theresa May did in March 2017. While legally entitled to do so, the success of this strategy likely depends on the prime minister offering compromises on Brexit (since Scotland voted to remain in the EU) or greater self-government. If the prime minister chooses to simply rely on the exercise of legal authority to maintain the territorial integrity of the UK, it could damage trust – and stability in Scotland.

3. Wales: why is support for independence rising?

In the period since the Brexit referendum, a variety of grassroots movements have emerged in Wales, calling for independence from the UK - culminating in the first ever march for independence in Cardiff in May 2019. A recent YouGov poll suggests a significant increase in the percentage of people in Wales identifying as "indycurious" – open to consider the option of Welsh independence.

While further investigation is needed to clearly identify the long-term significance of the rise of indycurious Wales, its initial results should not be ignored by May's successor. Wales has traditionally been the most reluctant of the three non-English parts of the UK to pursue a distinct path away from the UK centre. Therefore, the emergence of data showing that this is changing – and among Labour voters in particular – serves as a litmus test on the ascendance of regional dissatisfaction towards Westminster.

4. England: how to provide a political voice?

In 2000, Rick Rawlings, professor of public law at UCL, observed that England had become "the spectre at the feast" in the UK devolution process. Over the last two decades, England has failed to receive a level of devolution comparable to the rest of the UK. While processes have been introduced to provide English MPs with a voice on England only matters, and certain English local authority areas have received additional powers as combined authorities, England remains the most centralised of the UK's four component nations.

Yet, despite its centralised character, recent research suggests that distinct circumstances emerging in England are now coming to play an important role in the UK's constitutional and political landscape. Most notably, public attitudes in England show higher levels of euroscepticism, compared to the rest of the UK, and anxiety over England being left behind by the UK devolution process. To successfully navigate these issues, the future prime minister needs to be prepared to address questions on England which may require difficult UK-wide compromises.

5. How do you reunite a disunited kingdom?

This issue facing the next prime minister relates to the UK-wide territorial constitution.

With devolution now in its 20th year, much has changed in the UK's internal architecture. This has caused an ideological gulf to open up between the UK and devolved governments in their interpretation of the territorial constitution. On the one hand, the UK government maintains the vision of the UK as a "union of solidarity". On the other, the devolved administrations - particularly Scotland and Wales - see the UK as a multi-national state, whereby each of the four component nations has an equal say on its future development.

The realities of these divisions have been highlighted, for example, in the disputes over the extent to which the UK government had to consult the devolved administrations on the decision to trigger Article 50 to begin Brexit negotiations. Here, the legal superiority of the UK parliament triumphed to the detriment of the security and trust of the devolved administrations.

If the next prime minister is to safeguard the territorial integrity of the UK, and return it to a period of constitutional stability, they must work to find a compromised position in the interpretation of the UK's evolving constitutional dynamics.

The constitutional challenges facing the next prime minister are significant. Brexit has both highlighted pre-existing fault-lines in the territorial constitution, and exacerbated many of the issues which have emerged over the last two decades of devolution. The task for the next prime minister is to find solutions to these issues where their predecessors have failed.

Gareth Evans is lecturer in law at Staffordshire University This article is republished from The Conversation under a Creative Commons license.