Speaking to The Independent, the former Conservative prime minister expanded on his dramatic warning that MPs have “a duty” to act, if dishonesty is shattering public trust in politics.
If Mr Johnson wins a likely no-confidence vote among his MPs, the only route to removing him will be a confidence vote on the Commons floor – which can only succeed if several dozen Tory MPs rebel.
Sir John declined to pre-judge the police inquiry into the No 10 parties, arguing it is better to “wait and see” if the prime minister has “given an accurate version of what happened”.
But, asked if Tory MPs should “put country before party”, if Mr Johnson is shown to have lied to parliament, he replied: “I hope Conservative MPs would always put country before party, in all circumstances.”
Asked, by The Independent, if that included potentially rebelling against the government in a Commons vote of confidence, Sir John repeated: “In all circumstances.”
Although defying Mr Johnson in that way would ordinarily end an MP’s career – preventing them being a Tory candidate at the next election – some Conservatives believe it is a plausible scenario.
Some unhappy Tories have already announced they are quitting the Commons and, following the Brexit battles of the last parliament, the whip was later restored to some rebels.
One former Tory minister said recently: “The next Conservative leader could decide to bring an MP back in those circumstances, if they were seen to have acted in the national interest.”
Sir John spoke after intervening in the Partygate controversy by arguing it is clear that Mr Johnson broke lockdown rules at the No 10 parties now under investigation.
In a speech, he condemned the “unbelievable” excuses “dreamed up” to deny that rules were flouted – warning they are weakening public’s trust in politics.
Sir John argued that “deliberate lies to parliament have been fatal to political careers – and must always be so”, making clear Mr Johnson must quit or be forced out, if he misled the Commons.
“At No10, the prime minister and officials broke lockdown laws. Brazen excuses were dreamed up. Day after day the public was asked to believe the unbelievable,” he said.
“The lack of trust in the elected portion of our democracy cannot be brushed aside. Parliament has a duty to correct this. If it does not, and trust is lost at home, our politics is broken.”
Tory MPs are still awaiting the full Sue Gray report and Met investigation into No 10 lawbreaking, before deciding whether to demand a no-confidence vote.
The crisis deepened for the prime minister overnight, when the police sent out 50 questionnaires to people believed to have broken rules – including to him, it is believed.