A leading lawyer in the legal fight against Boris Johnson’s prorogation of parliament has said he may need to leave the country over death threats triggered by the supreme court case that ruled the suspension “unlawful, void and of no effect”.

Jolyon Maugham QC has revealed that his local police and crime commissioner was sufficiently concerned by recent threats against his life that he was advised to buy a stab vest, while a former counter-terrorism expert was ordered to install a panic button in Maugham’s home. High-end security equipment is also installed outside his home.

Local police have been told to frequently drive past Maugham’s house to ensure the family are safe. Dale Vince, Maugham’s co-litigant and millionaire owner of the electricity company Ecotricity, is paying for the law firm Schillings to monitor the internet and the dark web for specific threats against Maugham.

Maugham, a barrister and founder of the Good Law Project, said he had been advised by security experts to hire a bodyguard for key public events.

Quick guide The six key paragraphs in the supreme court's verdict Show Hide That the court judgment was not about Brexit “It is important, once again, to emphasise that these cases are not about when and on what terms the United Kingdom is to leave the European Union. They are only about whether the advice given by the prime minister to Her Majesty the Queen on 27 or 28 August, that parliament should be prorogued from a date between 9 and 12 September until 14 October, was lawful and the legal consequences if it was not.” That the court had the right to act “The first question is whether the lawfulness of the prime minister’s advice to Her Majesty is justiciable. This court holds that it is. The courts have exercised a supervisory jurisdiction over the lawfulness of acts of the government for centuries.“ That the prorogation was not ‘normal’ “It prevented parliament from carrying out its constitutional role for five out of the possible eight weeks between the end of the summer recess and exit day on 31 October. Proroguing parliament is quite different from parliament going into recess. While parliament is prorogued, neither house can meet, debate or pass legislation. Neither house can debate government policy. Nor may members ask written or oral questions of ministers or meet and take evidence in committees…This prolonged suspension of parliamentary democracy took place in quite exceptional circumstances: the fundamental change which was due to take place in the constitution of the United Kingdom on 31 October. Parliament, and in particular the House of Commons as the elected representatives of the people, has a right to a voice in how that change comes about. The effect upon the fundamentals of our democracy was extreme.” That Boris Johnson’s advice to the Queen was unlawful “The court is bound to conclude, therefore, that the decision to advise Her Majesty to prorogue parliament was unlawful because it had the effect of frustrating or preventing the ability of parliament to carry out its constitutional functions without reasonable justification.“ That parliament has not been prorogued “This court has already concluded that the prime minister’s advice to Her Majesty was unlawful, void and of no effect. This means that the order in council to which it led was also unlawful, void and of no effect and should be quashed. This means that when the royal commissioners walked into the House of Lords it was as if they walked in with a blank sheet of paper. The prorogation was also void and of no effect. Parliament has not been prorogued. This is the unanimous judgment of all 11 Justices.” What happens next? “It is for parliament, and in particular the Speaker and the Lord Speaker, to decide what to do next. Unless there is some parliamentary rule of which we are unaware, they can take immediate steps to enable each house to meet as soon as possible. It is not clear to us that any step is needed from the prime minister, but if it is, the court is pleased that his counsel have told the court that he will take all necessary steps to comply with the terms of any declaration made by this court.” The full judgment and the summary judgment can be downloaded from the Supreme court website.

“I have bought a stab vest and will hire personal security too. I’m really worried,” he said. “I was slightly embarrassed, getting this level of attention from a police force that, like all others, is financially stretched but the detective constable insisted that it’s proportional and necessary: he said there’s a lot of lone wolf, far-right threats around at the moment and I need to be protected.”

The threats began during the court case this week, which led to the supreme court ordering MPs back to parliament. But police fears for Maugham’s safety increased after a journalist published his home address.

“Publishing my address has massively increased what is perceived to be the threat against my life,” he said. “It wasn’t helped by an incredibly provocative quote in the media by a No 10 source attacking the people bringing the legal case and threatening the judiciary.

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“Things are going to get an awful lot worse in six weeks because we have another legal case coming up in Scotland which seeks to compel Boris Johnson to send the extension letter,” he added.

Maugham fears “the finger will be pointed at me, in particular, because I’m the most vulnerable of the three litigants”, the other two being Vince and Joanna Cherry MP.

“I expect things to get very ugly when this next court case begins and am thinking of leaving the country temporarily if the police and I judge the atmosphere to worsen,” he said. “I’m fortunate in that I can work away from the office.”

The group of Maugham, Vince and Cherry, will ask judges in Scotland to rule that they will ask Brussels for a Brexit extension in the prime minister’s name if Johnson refuses to do so.

The British prime minister has said he would rather be “dead in a ditch” than ask for a Brexit extension beyond 31 October, even though an act of parliament now requires him to do so if he fails to agree a new deal with the EU next month.

Maugham has also sought legal advice that he said proves that, despite fears from MPs, the Fixed-term Parliament Act (FTPA) does not inevitably lead to the dissolution of parliament.

“A vote under the FTPA is not a necessary precondition to the replacement of the existing prime minister with another,” he said.

“There are other mechanisms for Boris Johnson’s replacement, mechanisms that do not carry with them the risk of a dissolution.

“MPs can have two free hits at replacing Boris Johnson – first, asking whether there might be a majority for Jeremy Corbyn and, if not, second, whether there might be a majority for someone else. And if there has been no vote of no confidence under the FTPA then there is no risk of a general election if MPs cannot cohere around an alternative.”

Maugham’s local police and crime commissioner said the issue was an operational matter and made no comment.