Johnson argues that he asked the Queen to prorogue — or suspend — the lower house in order to introduce a new legislative agenda. But critics accuse him of attempting to stymie debate and push through a no-deal Brexit as the UK nears an October 31 deadline.
Last week, Scotland's highest court ruled that Johnson's suspension was an unlawful attempt to silence debate on Brexit in the House of Commons. An English court had earlier ruled that the prime minister's prorogation of parliament was legal.
Read more: Brexit: Does Boris Johnson's track record explain the man?
Crucial decisionObservers have described the court case as one of the most challenging to reach the Supreme Court in centuries, saying it could have significant consequences.
"If parliament is prorogued with no remedy available then the balance of power is tipped far too heavily to the executive," Sionaidh Doughlas-Scott, law professor at London's Queen Mary University, told Agence France-Presse news agency.
If the court rules against Johnson, it is likely that Parliament would reconvene. However, it is unclear when that would take place.
The court hearings could run through to Thursday with a ruling possible by as early as Friday.
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ls/stb (Reuters, AFP)