Article 50 letter hand delivered to European Union by British ambassador, initiating two-year countdown to EU exit.
British Prime Minister Theresa May has invoked Article 50 of the Lisbon Treaty to leave the European Union and now has two years to finalise the break with what will become a 27-nation bloc.
The UK must agree on new terms with the EU in a vast number of areas, including freedom of movement, trade, security and financial regulations.
Below are just some of the subjects that the UK and EU’s top diplomats will be discussing.
One of the key reasons for the Brexit vote was immigration, particularly concerns among some segments of the UK over the purported effect migrants from Eastern European countries were having on British services, such as healthcare and education.
Research by the London School of Economics suggests EU migrants have little effect on the unemployment rate of British nationals, put relatively little strain on services, and actually bring with them a number of economic benefits.
The UK is, therefore, unlikely to demand a full halt on immigration but will seek to balance the economic benefits it brings with rising anti-immigrant sentiment in the country.
The UK’s Home Secretary Amber Rudd has promised that Brexit will change freedom of movement “as we know it”, meaning significant change from the current status quo in which Britons can live, work and do business in EU states without visas and with the same rights locals of those countries have and vice versa.
British diplomats will be hoping to maintain as many of these privileges as possible for their nationals while satisfying demands for restrictions on inward migration of EU nationals into the UK.
An end to freedom of movement will result in the UK’s exclusion from the single market, a tariff-free trading zone that currently spans 32 countries.
The European Council president, Donald Tusk, has warned the UK that it cannot have “a la carte” access to the parts of the EU that it wants while rejecting parts it doesn’t.
In January, May seemed to accept as much, equating membership of the single market with EU membership itself.
A complete redrawing of trade relations could take years and will involve separate negotiations for various different industrial sectors.
A key area of concern will be the UK’s border with the Republic of Ireland, as border communities and even unionists in Northern Ireland fiercely oppose the introduction of a physical border between the two.
Ending unimpeded travel between the two will particularly heighten tensions between the UK government and Republican communities, with many openly mulling renewed attempts at unifying the island.
About 1.2m Britons live in another EU state and an estimated 3.2m EU nationals live in the UK.
May has avoided giving guarantees on what happens to those living in the UK, pending what EU states decide on British nationals living in their countries.
Perhaps the one area the EU and the UK will quickly find common ground is in security and intelligence cooperation.
European states face common threats in the form of groups, such as the Islamic State of Iraq and the Levant (ISIL) and in countries, such as Russia, which has sought to influence the internal politics of several EU states.
Passporting rights allow businesses registered in one member of the single market to operate in another without any further registration.
Failure to secure passporting rights would force companies based in the UK to register separately in a European single market country. The process for doing so would be long and costly.
That could lead to them uprooting from the UK altogether to access the larger EU market to the detriment of London’s status as a global financial hub.