Story highlights Appeal on whether government can trigger Article 50 without vote

Lower court ruled that members of parliament must be given a say

Hearing is expected to last four days, decision due in early 2017

London (CNN) The UK's Supreme Court has begun hearing an appeal over whether members of parliament must approve the decision to trigger Article 50, sparking Britain's withdrawal from the European Union.

The Brexit process was thrown into confusion in November when the High Court sided with campaigners who had argued that the government must seek the support of MPs.

Prime Minister Theresa May has said she wants to trigger Article 50 by the end of March 2017, paving the way for formal negotiations that would likely see Britain leaving the EU in 2019.

But with little agreement on what UK-EU relations would look like post-Brexit, discussions on the issue have already proved complex and bad-tempered.

All 11 judges will hear the four-day case -- the first time this has happened since the Supreme Court was established in 2009 -- before giving their ruling in early 2017.