The former director of public prosecutions has told Sky News that Boris Johnson could go to prison if he refuses to delay Brexit in the face of court action.
The prime minister has said he will not agree an extension, despite parliament passing a law forcing him to do so.
Lord MacDonald, who held the senior prosecutor post between 2003 and 2008, said legal action would mean a court ordering that “the law should be followed”.
“A refusal in the face of that would amount to contempt of court which could find that person in prison”, he said.
The cross-bench peer said this was “not an extreme outcome” as it was “convention” that individuals who refuse to “purge their contempt” are sent to prison.
However, it is also possible that a court could demand another figure in government authorises the delay.
Lord MacDonald added Mr Johnson and his senior adviser Dominic Cummings might have found a way not to have to delay Brexit, saying: “I expect they have some ideas that they’re working on.”
Dominic Grieve, an MP expelled from the Conservatives this week for backing an anti-no-deal law and former attorney general, told Sky News that Mr Johnson was acting like a “spoilt child having a tantrum”.
If he refuses to obey the law he will be “sent to prison for contempt,” the Beaconsfield MP said.
Former Supreme Court judge Lord Sumption also told Sky News there would be “plenty of ways” to enforce the law.
He said MPs opposed to no-deal could apply for an injunction ordering Mr Johnson to authorise a delay.
If the prime minister still refused to comply, a judge could make an order demanding that a government official sign off the extension “in place of the prime minister”.
“He’s not going to be marched off to Pentonville Prison… it’s much less dramatic than all that”, he added.
However, the author and historian told Sky News he thought it was unlikely to get to that point as senior civil servants would refuse to co-operate with a prime minister who was wilfully breaking the law.
“He won’t get any co-operation, apart from the fanatics around him… the attorney general won’t sit there quietly while this happens.
“If he was to do something as foolish as that, he would be on his own, maybe accompanied by Dominic Cummings”, he said.
It comes after the so-called “Rebel Alliance” of opposition MPs and those ousted from the Conservatives threatened legal action to ensure the prime minister complies with the law.
A source close to them said it was a “necessity” because Mr Johnson had “in no uncertain terms” indicated he had “no intention of complying with the law”.
Their cross-party bill – which requires the prime minister to ask Brussels to delay Brexit until January unless a deal is agreed by 19 October or MPs back no-deal – is expected to become law on Monday.
Asked by reporters on Friday if he would follow the legislation when it becomes law, Mr Johnson said: “I will not. I don’t want a delay.”
He followed up that pledge in an email to Tory members saying parliament “just passed a law that would force me to beg Brussels for an extension to the Brexit deadline”, adding: “This is something I will never do.”
Chief Whitehall no-deal planner Michael Gove confirmed earlier this week the government would comply with the legislation.
David Gauke, one of 21 MPs expelled from the Conservatives for backing the anti-no-deal bill and justice secretary until the start of the summer, has written to the attorney general to ask him to confirm the government believes in the rule of law.