The United Kingdom faces an uncertain future Jeff J Mitchell / Getty LONDON — The United Kingdom faces an uncertain future after Brexit, with growing threats that both Scotland and Northern Ireland could break away.

The independence movement in Scotland and increasing uncertainty over the post-Brexit border between Northern Ireland and the Republic could combine to end centuries of political union between the nations of the UK.

Theresa May's decision to leave the single market and customs union has led to increased political tension in Northern Ireland.

In recent weeks the Irish government has reportedly moved towards backing a new sea border between mainland Britain and Northern Ireland after Brexit in order to prevent smuggling.

This is in opposition to UK government proposals for some form of technological border, which the Irish government believes would be unworkable.

The row risks derailing the Irish peace process, which is partly dependent on the free movement of people and goods between the two countries.

Tension on the issue was heightened on Wednesday when an Irish parliamentary committee published plans for Northern Ireland and the Republic to reunite.

The proposals, which were set out in preparation for Brexit, envision a cross-border referendum on a united Ireland taking place over the coming years.

Polls in recent years have shown majority opposition in Northern Ireland to unification. However, any attempt to re-open the question on either side of the border would prove hugely divisive and could further threaten peace.

Disunited Kingdom

Britain's decision to leave the European Union has also led to renewed calls for a second Scottish independence referendum.

Scottish first minister Nicola Sturgeon has pushed, so far unsuccessfully, for a new poll before Britain leaves the EU in 2019.

Polls continue to show majority opposition to reunification. However, the Scottish government remains committed to holding another referendum. With leaving the EU remaining opposed by the majority of Scottish people, a botched, or failed Brexit, could easily cause the question of independence to rear its head again.

Concern about the kingdom splitting is high. Polling last year found that 62% of Scottish people and 53% of all UK voters, believed the country will fracture over the next ten years.

Comres / BBC poll: The United Kingdom will no longer exist as either Scotland, Wales, or Northern Ireland will be independent: