Senior judges and lawyers have warned that Boris Johnson would be in contempt of court if he applied for an article 50 extension while simultaneously trying to get the EU to reject it.

Reports in the Daily Telegraph suggested the prime minister had drawn up plans to “sabotage” parliament’s efforts to force through a Brexit extension to prevent the UK leaving the bloc without a deal.

He was said to be considering sending an accompanying letter to the EU alongside the request to extend article 50, which would say the government did not want any delay to Brexit.

Ken Macdonald, the former director of public prosecutions, warned that such a plan would be breaking the law. He said: “The act is quite clear in what it requires the prime minister to do. If he sends a side letter to the EU that deliberately conflicts with that requirement, he is deliberately in breach of the law.”

Lord Macdonald added: “This is the sort of disreputable wheeze that might appeal to an adviser in Downing Street, but it is unlikely to appeal to a court. The predictable result would be an injunction requiring the prime minister to do what should be first nature to him: obey the law.”

The Criminal Bar Association (CBA) condemned suggestions that the prime minister was planning to ignore legislation designed to stop a no-deal Brexit. The group’s chair, Caroline Goodwin, said: “We cannot expect people not to rob, rape and murder when a government declares it may break the law. We cannot lay rape to the rule of law.”

Goodwin added: “As the CBA our role is not to say ‘remain’ or ‘leave’ but part of our role is to explain the law – criminal law – and play our part in upholding the rule of law.”

Baroness Helena Kennedy QC said: “Courts are not going to entertain skulduggery and that is what the ruse of the second letter is. One of the fundamental principles of the rule of law is that it applies to everyone, including prime ministers. This law has been passed by parliament. Those who defy the law become accountable. Mr Johnson would be in contempt of court if he tried to finagle his way out of the obligations set down in the act.” She added that her clients “often see sense when jail beckons”.

Jonathan Cooper, a barrister at Doughty Street Chambers, said unless Brexiters could find a way to challenge the act preventing no deal, the government had to comply.

“There is only one building block to the UK system of government and that’s the sovereignty of parliament. That means parliament trumps the prime minister. The EU is required to take account of the UK’s constitutional framework. In deciding whether to give an extension, the EU will be guided by an act of parliament and not the prime minister. If the prime minister chooses to undermine parliament, the courts will step in and if need be hold the prime minister in contempt.”

Susan D Shaw, managing partner at Living Law, based in Scotland, said recent events should be “ringing the alarm bells across the country for every citizen”, regardless of their views on Brexit. “The rule of law is being seriously eroded with each and every day that now passes and with it all of our hard-won rights as citizens. History shows us where this dangerous path leads.”

She added that Johnson’s actions had “tainted the history and stature of our world-leading institutions and are diminishing our standing in the global order. On any rational view he is simply unfit to hold high office.”

Shoaib M Khan, a human rights lawyer at SMK Law Solicitors, said: “The prime minister’s plan to somehow seek an extension yet not seek an extension is senseless, an affront to the rule of law and almost certainly unlawful. It would bring parliament, and indeed the country, into disrepute.”

Khan pointed to the recent announcement that the civil rights group Liberty has launched a judicial review case seeking to compel Johnson to comply with a bill. “The PM should immediately announce he will properly comply with the wording and spirit of the legislation, and refrain from further irresponsible attempts to play games with the future of this country,” he said.

Earlier on Monday, Jonathan Sumption, a former supreme court justice, said it would not be legal for the prime minister to ask for an extension while rubbishing the request at the same time.

Lord Sumption told BBC Radio 4’s Today programme: “The bill, or the act as it is about to become, says that he’s got to apply for an extension. Not only has he got to send the letter, he’s got to apply for an extension. And to send the letter and then try and neutralise it seems to me to be plainly a breach of the act.”

Sumption said he had read the bill and there was not “the slightest obscurity” about what the government was obliged to do. He said: “You have got to realise that the courts are not very fond of loopholes. They are going to interpret this act in a way that gives effect to its obvious purpose unless there’s something in the act that makes it completely impossible to do so and there isn’t.”

Sumption said Johnson would not only be in contempt of court if he failed to do what the bill stated, but would risk the resignation of the justice secretary, the attorney general, and other members of his cabinet.

He added there were “plenty of ways” in which this kind of obligation could be enforced. “An application will have to be made to the court for an injunction. The simplest way of enforcing the injunction would be for the court simply to direct an official to sign the letter on behalf of the PM and to declare that his signature was to be treated in every legal respect as equivalent to the prime minister’s,” he explained.