Story highlights Article 50 of Lisbon Treaty paves way for formal negotiations between EU and Britain

Once invoked, agreement on UK-EU relations has two-year deadline

(CNN) British voters opted out of the European Union in last year's historic referendum, but Brexit can't actually get underway until Article 50 of the Lisbon Treaty is triggered.

So what is Article 50?

The Lisbon Treaty, which came into effect in 2009, is one of two treaties that make up the constitutional foundations of the European Union. Within the Lisbon Treaty lies a 250-word, five-point framework, which outlines voluntary departure from the European Union.

Article 50 states: "Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements."

The article specifies that the exiting state must notify the European Council of its intent so formal negotiations for a withdrawal agreement can begin.

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