Parliament was prorogued, or suspended, on Monday until October 14, a move opponents argued was created to thwart their attempts to scrutinize his plans for leaving the European Union and allow him to push through a no-deal Brexit.
If Johnson fails to secure a deal he insists the country will leave anyway, to the outrage of many MPs who believe a "no deal" exit would bring huge disruption.
However, a government source told AFP that "nothing is changing" until the case was concluded.
A group of about 70 lawmakers is challenging the government's decision to prorogue, or formally shut down, Parliament, for five weeks until October 14 - just over two weeks before the U.K.is due to leave the EU.
A spokesman for No 10 said it was "disappointed" by the decision, and would appeal to the Supreme Court. That contradicts the prime minister's stated motive for the move - that his new government needs the time to lay out its agenda in a speech in the middle of next month. "Proroguing Parliament is the legal and necessary way of delivering this", said the spokesperson.
The Welsh Government in Cardiff intends to be a party to the Supreme Court case - it has opposed prorogation and supported Gina Miller's High Court challenge, which was rejected.
Opponents argue that Johnson is trying to evade democratic scrutiny.
"Boris Johnson as prime minister has tried to ride roughshod over this and the Scottish legal system has brought him to task".
Scotland's First Minister Nicola Sturgeon reacted to the accusations made by the Downing Street source - who accused the judges of political bias - saying it was "pitiful, pathetic and desperate". He added, "It was to be inferred that the principal reasons for the prorogation were to prevent or impede Parliament holding the executive to account and legislating with regard to Brexit, and to allow the executive to pursue a policy of a no-deal Brexit without further parliamentary interference".
The judges said the suspension was "null and of no effect", but referred the matter to Britain's Supreme Court for resolution.
And Lord Drummond Young said that the United Kingdom government had failed to show a valid reason for the prorogation, adding: "The circumstances, particularly the length of the prorogation, showed that the objective was to prevent such scrutiny".
"That is why I and others would have put the chances of the action succeeding in London as zero".
In a Facebook broadcast Johnson read out a submitted question from a voter, which said he was running an authoritarian government and asked "Why is home-grown authoritarianism better than European Union rule?"
Britain's main opposition Labour Party should campaign against Brexit and push for a referendum to reverse its planned departure from the European Union before any election, deputy party chief Tom Watson, is due to say on Wednesday.