Boris Johnson would have no option but to recall MPs to Westminster if the supreme court rules he misled the Queen, senior legal sources told the Observer yesterday.

There is a growing belief in the legal community that the court will find against the government when it hands down its momentous verdict on Johnson’s decision to prorogue parliament.

The prospect of the court finding against the prime minister has left the UK heading towards a “constitutional eruption of volcanic proportions”, according to another senior legal figure who asked not to be named. He said he also believed the case would go against the government.

Before the case, few thought the court would determine that Johnson’s advice to the Queen to suspend parliament for five weeks would be found unlawful. But over the course of the three-day hearing opinion has dramatically shifted.

“The dominant feeling among informed observers is that the government is on the ropes and it’s going to lose,” said Philippe Sands QC, professor of law at University College London.

A panel of 11 justices heard appeals arising out of two separate legal challenges in England and Scotland, in which leading judges reached different conclusions.

The court’s president, Lady Hale, has said the judges hope to give their decision early this week.

“We are solely concerned with the lawfulness of the prime minister’s decision to advise Her Majesty to prorogue parliament on the dates in question,” she said. “As we have heard, it is not a simple question, and we will now carefully consider all the arguments that have been presented to us.”

Experts were quick to identify three aspects of the case that should trouble the government. First, the judges spent a large portion of their time exploring possible remedies – what they might determine must happen if they find against the prime minister. In other cases, judges seldom devote effort to discussing remedies if they are not seriously considering finding in favour of the complainant, legal sources say.

Second, Hale’s forensic questioning of the government’s lawyer, Lord Keen, suggested she was dubious about the arguments he was presenting. Third, the fact that the court has yet to deliver a decision should send alarm bells ringing. In the words of one legal expert, “the fact they haven’t given a decision now with reasons to follow would tend to suggest the government is going to lose at some level. If they were going to find for the government, why bother stringing it out?”

Johnson has pledged to abide by the ruling, but Keen refused to rule out that the government might suspend parliament again ahead of the Brexit deadline next month. Whatever the court decides will prove explosive. Lawyers opposing the government have claimed that Johnson’s action was illegal, saying the “exceptionally long” prorogation was intended to “silence” parliament.

But Keen, on behalf of the government, submitted that the courts “must not cross the boundaries and intrude upon the business of parliament”.

Sands said the crucial question was what remedy the court would suggest if it did find against the government. Much will rest on the detail in the court’s decision and how it is framed. If the court determines that Johnson misled the Queen, that would make his position untenable, according to Dominic Grieve, the former Conservative attorney general.

But the court might choose to frame its verdict around whether Johnson’s decision stymied the passage of legislation through parliament, which is potentially less incendiary.

If the court does find against the government, its lawyers will be scrutinising the decision to see if they can go back and order a second prorogation on other grounds.

It means that this week’s decision is unlikely to be the end of the matter, whatever the justices decide.