What is the freedom of movement in the EU?
It is one of the three basic freedoms in the single market of the European Union: along with freedom of goods, freedom of capital and services, there is also the freedom of movement.
As a European citizen, former EU Justice Commissioner Viviane Reding observes, you can travel across Europe - from Vilnius in Lithuania to Valencia in Spain - without once stopping at a border.
Any EU citizen, from the 28 - although soon-to-be 27 - member countries, can move to and remain in another EU country for up to three months. Students can stay for the duration of their studies, but must prove they have sufficient financial support. Other EU citizens who wish to stay longer than three months, according to Open Society Foundations, must have comprehensive sickness insurance and prove that they have financial resources to support themselves.
Freedom of movement, however, was "not carved on stone tablets" and was initially restricted to workers, according to the Guardian. It wasn't until 1993 when free movement of workers became free movement of citizens.
Around 3.2 million EU citizens live in the UK, making up 5 per cent of the overall population. An estimated 916,000 are Polish nationals, Full Fact notes, followed by 332,000 Irish nationals and 233,000 Romanians as the largest single nationalities from the rest of the EU living in the UK.
Meanwhile, around 1.2 million UK citizens live in the EU, according to 2015 estimates from the United Nations. Around 900,000 of those Brits abroad are long-term residents.
As a fundamental right for members of the EU, it will be one of the first things discussed in the Brexit negotiations. Donald Tusk has warned that the Government must settle the issues of “people, money and Ireland” before negotiating its future relationship with the EU. The European Council President said his approach was “not only a matter of tactics, but - given the limited time-frame we have to conclude the talks - it is the only possible approach”.
Prime Minister Theresa May has suggested that free movement of people from the EU to the UK could be extended after Brexit.
She said: "You've used the phrase transitional phase; I have used the phrase implementation period.
"Once we've got the deal, once we've agreed what the new relationship will be for the future, it will be necessary for there to be a period of time when businesses and governments are adjusting systems and so forth, depending on the nature of the deal - but a period of time when that deal will be implemented."