Leading judges are expected to announce their decision on Friday on the latest legal attack brought over Prime Minister Boris Johnson's decision to suspend Parliament for five weeks.
Lord Chief Justice Lord Burnett and two other judges at the High Court in London have been urged to find that Mr Johnson's August 28 advice to the Queen to prorogue Parliament for an "exceptional" length of time was an "unlawful abuse of power".
After hearing submissions on Thursday on an urgent judicial review application brought by businesswoman Gina Miller, Lord Burnett announced that the hearing would be adjourned until 10am on Friday.
He said: "If we are able to do so we will announce our decision, but the written reasons will follow as soon as we are able to prepare them."
The case brought by Ms Miller, who successfully challenged the Government at the High Court in 2016 over the triggering of the Article 50 process to start the Brexit countdown, is supported by a number of other parties, including former prime minister Sir John Major.
The action was contested by the Prime Minister, whose lawyer argued that the advice given to the Queen was not unlawful and that, in any event, Ms Miller's claim was "academic".
Lord Pannick QC, representing Ms Miller, told the packed court: "Our case is that the Prime Minister's advice to Her Majesty to prorogue Parliament for a period of five weeks is an unlawful abuse of power."
Lord Pannick told Lord Burnett, sitting with Master of the Rolls Sir Terence Etherton and President of the Queen's Bench Division Dame Victoria Sharp, that the reason given by Mr Johnson for suspending Parliament, to introduce a new programme of legislation, did not require a five-week suspension.
He said the decision to prorogue Parliament for that length of time was "fatally infected by the Prime Minister's failure to understand that Parliament is sovereign and that Parliament should be allowed to perform its functions in relation to potential legislation".
In submitting that the judges should reject Ms Miller's arguments, Sir James Eadie, on behalf of Mr Johnson, said: "The exercise of this prerogative power is intrinsically one of high policy and politics, not law."
Arguing that the claim was "academic", he pointed out that each House of Parliament will sit before the UK leaves the EU on October 31 "and may consider any matter it chooses".
He told the court: "The prorogation does not prevent Parliament from legislating on any matter it wishes. Parliament is capable of legislating at pace if it chooses to do so."
The hearing in London came the day after Mr Johnson fought off a similar action in Scotland.
Whatever the outcome of the challenges against the decision to prorogue Parliament, it is likely that the dispute will end up at the UK's highest court, the Supreme Court has set aside September 17 for any possible appeals.