Supreme Court judges will rule early next week on whether British Prime Minister Boris Johnson's decision to suspend parliament in the run-up to Brexit was lawful.

"We know that this case must be resolved as quickly as possible, and we hope to be able to publish our decision early next week," judge Brenda Hale said as the third and final day of hearings wrapped up.

Mr Johnson has suspended parliament for five weeks, with MPs only allowed to return on 14 October, a fortnight before Britain's planned exit from the European Union on 31 October.

The Conservative leader, who took office in July, insists it was a routine move to allow his government to launch a new legislative programme next month.

But critics accuse him of trying to silence critical MPs at a crucial time, with Britain's exit terms and departure date still uncertain.

"I must repeat that this case is not about when and on what terms the United Kingdom leaves the European Union. The result of this case will not determine that," Judge Hale said in brief closing remarks.

"We are solely concerned with the lawfulness of the prime minister's decision to advise her majesty (Queen Elizabeth II) to prorogue parliament on the dates in question. 

"As we have heard, it is not a simple question and we will now consider carefully all the arguments which have been presented to us."

Eleven of the Supreme Court's 12 judges heard appeals against two conflicting lower court decisions on Mr Johnson's move, involving multiple plaintiffs.

Scotland's highest civil court found the suspension was unlawful, but the High Court in England said it was not a matter for judges to intervene in.

Mr Johnson insists Britain must leave the EU with or without a divorce deal with Brussels, but MPs have legislated to force him to delay Brexit if he has not reached an agreement in time.

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Former British prime minister John Major believes Boris Johnson's prorogation decision was motivated by his "political interest" in closing down parliament.

Mr Major is an intervener in the unprecedented case at the Supreme Court.

He is supporting the appeal brought by campaigner and businesswoman Gina Miller against an earlier ruling by the High Court in London that the prorogation was "purely political" and not a matter for the courts.

Gina Miller at the UK Supreme Court

Earlier, Mr Major's barrister Lord Garnier said the intervention was "nothing to do with the arguments for or against Brexit".

In written submissions which were also provided to the court, Lord Garnier said the former prime minister is of the view the "inference was inescapable" that Mr Johnson's decision was "motivated by his political interest in ensuring that there was no activity in parliament during the period leading up to the EU Council summit on October 17 and 18".

Lord Garnier, who served as solicitor general under former prime minister David Cameron, added: "One of the central points of the present case - and the reason why these proceedings are necessary at all - is that the power of prorogation subverts the possibility of control by political means.

"Its effect is to deprive parliament of a voice throughout the period of the prorogation.

"There is no possibility of political control except in the limited sense that a prime minister who exercises the power in a damaging way might face political consequences at some later date, when Parliament is permitted to reconvene.

"But where the effect of the prorogation is to prevent parliament from discharging its role during a time-critical period, there is no possibility of meaningful political control of that decision until after the damage has been done."

Lord Garnier said it remains "genuinely unclear" whether Mr Johnson disputes that he was motivated by political interest because no witness statements have been provided.

He added that the submissions made on the Prime Minister's behalf at the High Court "studiously avoided committing to any clear position on the issue".

The barrister said the Supreme Court is under no obligation to approach the case on the "artificially naive" basis that the documents submitted on Mr Johnson's behalf should be "assumed to be entirely accurate and complete when even members of the Cabinet do not appear to believe them".

In a witness statement prepared for the High Court hearing, Mr Major said it was "utterly unacceptable" for the government to "seek to bypass" parliament because it does not agree with the proposed course of action on a certain policy.

The statement read: "I served in parliament for over 20 years both as a backbench MP and as a government minister at Cabinet and more junior levels.

"I was of course Prime Minister for nearly seven years and am very proud to have been in the Commons and a minister.

"I have huge admiration for our parliament and am a keen supporter of its rights and duties.

"I cannot stand idly by and watch them set aside in this fashion.

"I appreciate that this is not the government's stated intention for proroguing Parliament, but for the reasons set out in this statement, the inescapable inference to be drawn is that the prorogation is to prevent parliament from exercising its right to disagree with the government and to legislate as it sees fit."

The panel of 11 justices also heard submissions on behalf of the Welsh and Scottish governments and Northern Irish campaigner Raymond McCord.