The former Conservative prime minister condemned the Tory leadership favourite for failing to rule out prorogation – which he likened to the actions of Charles I in the 1640s.
Sir John said Tory Brexiteers were guilty of championing parliamentary sovereignty “except when it is inconvenient for Mr Johnson”.
“I for one would be prepared to seek a judicial review to avoid parliament being bypassed,” he threatened – predicting “a queue of people” would come forward.
The warning came after Mr Johnson used a live TV debate to double down on his threat to shut down parliament if necessary, insisting that he was “not going to take anything off the table”.
“I think it is absolutely bizarre at this stage in the negotiations for the UK yet again to be weakening its position,” he said – vowing again to meet the 31 October deadline.
In contrast, his rival Jeremy Hunt said: “When that has happened in the past and parliament was shut down against its will, we actually had a civil war,” adding: “So my answer is no.”
A senior source close to the former foreign secretary hit back immediately, telling a BBC reporter that Sir John had “gone completely bonkers” in making his threat.
Speaking to BBC Radio 4’s Today programme, Sir John said: “I think the idea of proroguing parliament is utterly and totally unacceptable from any British parliamentarian.”
He pointed out that such action by Charles I “didn’t end well for him” and therefore “shouldn’t end well” for Mr Johnson either.
Sir John argued prorogation would thrust the Queen into the “midst of a constitutional controversy”. But he continued: “The prime minister’s advice to the Queen can, I believe, be challenged in law.”
Defending his stance in the TV debate, Mr Johnson argued: “People can see that we are fatally losing trust of the electorate, and they can see that we asked people three years ago whether they wanted to leave the EU, they returned an overwhelming verdict that they wanted to come out. Parliament has failed to do that for three years.
“If we now fail again, if we fail again, I am afraid we will not win back the hundreds of thousands of voters who are currently deserting us for other parties, and that is how to lose the forthcoming election.”
Theresa May’s effective deputy David Lidington said that proroguing parliament in order to prevent MPs voting on a no-deal Brexit would be a “constitutional outrage”.
Asked whether suspending sittings in these circumstances would be acceptable, Mr Lidington told the Commons Exiting the EU Committee: “No. All members in this place have the right to express their own opinions. I think it would be a constitutional outrage.”
Sir John also stood by Sir Kim Darroch – the UK ambassador to the US under attack from Donald Trump after his criticism of Washington was leaked – suggesting Mr Johnson was ready to “throw him to the wolves”.
He said the ambassador was doing “precisely what he was instructed to do”, adding: “We really cannot have our ambassadors chosen by host governments. He should not be forced out of his office.”
Sir Kim later resigned from his post.
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