Lord Keen QC says such a question would «have to be addressed by the decision maker», as the Supreme Court showdown begins.
A government lawyer would not rule out Boris Johnson trying to close parliament for a second time during a Supreme Court hearing.
Lord Keen QC was outlining the government’s appeal against a Scottish court’s ruling that the prime minister suspending, or proroguing, parliament for five weeks was unlawful.
Mr Johnson made the move at the end of August, asking the Queen to approve a suspension from the week beginning 9 September to 14 October.
The PM said this was so he could set out his government’s legislative agenda at the start of the new session of parliament next month — but opponents argue he wanted to curtail scrutiny of his Brexit strategy.
Lord Pannick QC, the lawyer representing activist Gina Miller, earlier told the hearing that Mr Johnson wanted to «avoid parliamentary control» and «silence» MPs in the run-up to Brexit.
The UK’s highest court has begun a three-day hearing which will hear two separate challenges brought in England and Scotland over the legality of prorogation.
The High Court in London dismissed Mrs Miller’s case, ruling that the issue was «purely political» and not a matter for the courts.
However, the Court of Session in Edinburgh concluded that the PM’s move was unlawful, after a legal challenge was launched by a cross-party group of more than 70 MPs.
The Supreme Court will consider the two appeals and determine whether Mr Johnson’s advice to the Queen to suspend parliament is «justiciable» — capable of challenge in the courts.
If they do, the justices will then rule whether it was lawful.
The court will hear more submissions on Wednesday and Thursday, although it is not clear exactly when a ruling will be made.
The question of what the government will do if the Supreme Court rules against it was raised during proceedings.
Lord Keen said Mr Johnson would «take the necessary steps» to comply with the court’s ruling.
But when asked by Lord Kerr, one of the 11 justices hearing the appeals, what would happen if the court rules that the prorogation was unlawful, and whether parliament would be recalled, he responded: «It will be then for the prime minister to address the consequences of that declaration.»
Lord Keen, Scotland’s advocate general, continued: «I have given a very clear undertaking that the prime minister will respond by all necessary means to any declaration that the… prorogation was effected by any unlawful advice that he may have given.»
Lord Kerr then asked if it was the case that parliament could not be prorogued for a second time.
«I’m not in a position to comment on that,» Lord Keen replied.
«That will have to be addressed by the decision maker.»
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He added: «If the court finds it was unlawful, the prime minister will take the necessary steps to comply with any declaration made by the court.»
Referring to the Scottish case, Lord Keen said the courts «must not cross the boundaries and intrude upon the business of parliament».
He added that MPs would only lose «seven sitting days» because of prorogation, as it would have been in recess for party conference season for most of the five weeks.
Lord Keen said: «It is quite plain that the inner house [the Scottish court that ruled against the government], in addressing this issue and deciding that it could impugn the decision of the prime minister, was proceeding upon a fundamental misconception about how parliament works.»
Speaking before Lord Keen on Tuesday morning, Mrs Miller’s barrister said the case raises «fundamental questions of constitutional law».
Lord Pannick said: «The exceptional length of the prorogation in this case is strong evidence that the prime minister’s motive was to silence parliament for that period because he sees parliament as an obstacle to the furtherance of his political aims.»
He said it was «remarkable» that the PM had not given a statement to the court explaining why he had advised the Queen to prorogue for an «exceptionally long period».
Lord Pannick argued that the High Court had «erred in law» in Mrs Miller’s case and that the Court of Session had come to the «correct» conclusion about Mr Johnson’s motives.
A crowd of around 40 protesters, holding signs saying «Defend democracy», «Reopen Parliament» and «They misled the Queen», remained outside throughout the hearing.
A smaller crowd of pro-Brexit demonstrators, including one who shouted «traitor» at Mrs Miller as she left, also gathered outside.
Lawyers for Mr Johnson will outline the PM’s case that his advice on the suspension was lawful from 10.30am on Wednesday.