Downing Street put out “misleading” statements about the prorogation of parliament and published excuses for Boris Johnson’s five-week suspension of the Commons that are “not the true reasons”, the supreme court has been told by a lawyer for the former prime minister John Major.

The extraordinary clash of evidence between a former Conservative prime minister and the current one surfaced on the third day of an emergency supreme court hearing before 11 justices about whether Johnson’s five-week prorogation of parliament was lawful.

Lord Garnier QC, a Conservative former solicitor general, intervened in the case on behalf of Major with a strongly worded attack on the trustworthiness of Johnson’s newly appointed administration.

Downing Street, Garnier said, had been “misleading” in releasing announcements during the summer that initially suggested reports of an extended prorogation were ‘entirely false’. “Its effect was plainly to mislead,” he added.

As to the cabinet minutes suggesting prorogation was necessary to prepare for a Queen’s speech and break for the conference season, Garnier said they were “not the true reasons”.

The attack on the veracity of government statements by Major came as lawyers for the prime minister submitted defiant legal advice, threatening to suspend parliament again if the supreme court ruled Johnson’s prorogation unlawful.

In a written submission, lawyers for the prime minster say that: “In the event that the court holds … that prorogation was unlawful, with the effect that parliament was not prorogued and remains in session [then] depending on the court’s reasoning it would still either be open or not open to the prime minister to consider a further prorogation.”

Q&A What is a Northern Ireland-only backstop? Show Hide The British government’s version of Brexit involves the UK ultimately leaving the single market and customs union, requiring the return of a range of checks on goods crossing the border between Northern Ireland and the Republic of Ireland. The “backstop” is intended as a standstill placeholder to ensure such checks do not have to be imposed between Brexit happening with a deal, and the start of a new free trade agreement yet to be negotiated between the UK and the EU. Theresa May's withdrawal agreement proposed keeping the whole of the UK in a shared customs territory with the EU during this period. An alternative idea involves only Northern Ireland staying in the EU’s customs territory. That would place a customs border in the Irish Sea. May described it as a threat to the constitutional integrity of the UK, but the new prime minister, Boris Johnson, has opened the current talks by proposing an all-Ireland agri-food zone. The suggestion is that he will seek to quietly build on that with further NI-only arrangements. Given an NI-only backstop was an EU proposal in the first place, the U-turn would be warmly welcomed in Brussels, although attempts to give the Northern Ireland assembly a veto on its continuation would not be acceptable, and the DUP would be unlikely to support the prime minister in such a move in parliament. If there is a no-deal Brexit, then there is no backstop. Daniel Boffey

The government statement also states: “If the advice is found to be unlawful, the prime minister cannot commit to take any more particular steps without knowing the terms of any declaration and the reasons for it.”

Mother of parliaments shut down by father of lies, supreme court told Read more

The government declined to release its legal note, entitled Further Submissions on Relief, which asserts that it retains the power to re-suspend parliament. The document was tweeted out by Jolyon Maugham QC, of the Good Law Project, after it had already been referred to in court. Ministers are entitled to reconsider decisions that are deemed by a court to have been unlawful. It is highly unusual, however, for the government to declare in advance of a judgment that it intends to make the same decision again.

In submissions to the high court earlier this month, Garnier obliquely compared Johnson to an estate agent, citing a 2009 New Zealand case of premium real estates involving a salesman who misrepresented a buyer as a genuine purchaser when in fact they wanted to resell the property for a quick profit.

“It could hardly be suggested that the duties of the prime minister to the monarch are less than those of an estate agent to a homeowner,” Garnier noted.

“Accordingly, if the court is satisfied that the prime minister’s decision was materially influenced by something other than the stated justification, that decision must be unlawful, irrespective of whether the unstated justification was itself legally impermissible.”

The supreme court was also told earlier that the government’s failure to consider in its internal discussions on the five-week prorogation what happens in Northern Ireland was “gross” and “unthinkable”.

Johnson’s “do or die” battleground language of heading for a no-deal Brexit, Ronan Lavery QC said, was being “played out” on what could become a “scorched earth policy” in Northern Ireland.

But Lavery, who represented the victims’ rights campaigner Raymond McCord whose son was murdered by loyalist paramilitaries in 1997, was repeatedly interrupted by justices on the supreme court who questioned the relevance of his evidence about the dangers of a no deal.

Lord Wilson, one of the justices, said he was “really worried” that viewers listening would gain the mistaken belief that the court was considering the virtues of Brexit. In a sharp judicial put down, Wilson told him: “Don’t abuse our politeness or Lady Hale’s [the president of the court’s] patience.”

He was supported by Lord Reed who told the court: “Whether or not the withdrawal from the EU is a good or a bad thing is not an issue before us.” Another justice questioned the relevance of evidence from the government’s Yellowhammer report which set out the impact of a no-deal Brexit.

Lavery told the court that McCord “speaks for the silent majority in Northern Ireland who want a peaceful and prosperous future”.

In written submissions to the court, Lavery explained: “The only reason that Northern Ireland is a part of the United Kingdom is by the consent of its people freely given in the 1998 Good Friday agreement referendum. Therefore, if the rule of law is being undermined by a prorogation with the illegitimate, unlawful or unconstitutional aim or purpose or facilitating a no-deal Brexit, the constitutional position of Northern Ireland itself in the United Kingdom is undermined by this prorogation.

“Withdrawal from the EU in terms that are harmful and oppressive to the people of Northern Ireland … firstly undermines the principle of consent of the people of Northern Ireland by preferring the interests of English nationalism over the safety and welfare of the people of Northern Ireland. Secondly, it is a breach of the terms of the Good Friday agreement which states that it would be wrong for there to be any constitutional change in Northern Ireland without the consent of its people.”

The Scottish government’s lord advocate, James Wolffe QC, argued that no substantive justification has been provided for the five-week prorogation which was unusually long.

The hearing continues.