Sir John Major has said it is clear Boris Johnson broke Covid laws and warned he must quit if it's found he lied to Parliament.
In a scathing attack on the Prime Minister, the Tory grandee suggested that MPs had a "duty" to restore trust in politics or risk breaking the British political system.
Sir John attacked Mr Johnson's "foolish" behaviour, which he warned was undermining democracy, and said: “Deliberate lies to parliament have been fatal to political careers – and must always be so”.
In a speech to the Institute for Government, he said: "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.
"Ministers were sent out to defend the indefensible – making themselves look gullible or foolish.
"Collectively, this has made the Government look distinctly shifty, which has consequences that go far beyond political unpopularity.
"No Government can function properly if its every word is treated with suspicion."
Mr Johnson, on a visit to Brussels, has refused to say whether he will resign if he is fined by police over the Partygate saga.
The PM and wife Carrie are expected to be among some 50 people quizzed by Scotland Yard over rule-breaking gatherings in Downing Street and Whitehall during the pandemic.
Asked at a press conference if he would resign if he received a fine, the PM said: "That process must be completed and I'm looking forward to it being completed and that's the time to say more on that."
But when asked if a Prime Minister should resign after breaking the law, Sir John said: "Of course that has always been the case".
Sir John also suggested MPs may need to put "country before party" in a Commons vote of confidence.
"I hope Conservative MPs would always put country before party, in all circumstances," he told The Independent.
It comes as Tory MPs consider whether to trigger a confidence vote in the PM who has been left badly bruised by the Partygate row.
Sir John warned his fellow Conservatives: "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."
In a scathing intervention, he added: "Trust in politics is at a low ebb, eroded by foolish behaviour, leaving a sense of unease about how our politics is being conducted.
"Too often, ministers have been evasive, and the truth has been optional."
In remarks apparently directed at Mr Johnson - who has repeatedly come under fire over his elastic relationship with the truth, he added: "Politicians are not “all the same”.
"And lies are just not acceptable... To imply otherwise is to cheapen public life, and slander the vast majority of elected politicians who do not knowingly mislead. But some do – and their behaviour is corrosive.
"This tarnishes both politics and the reputation of Parliament. It is a dangerous trend."
Sir John tore into the Government's attempts to tear up the standards rules to protect disgraced MP Owen Paterson, who resigned last year after breaching lobbying restrictions.
"He chose to ignore critical reports on his ministers; rejected advice from his independent adviser on ministerial standards - who resigned; and attempted - but failed - to overturn a unanimous Standards Select Committee Report that condemned the behaviour of a parliamentary colleague and friend," he said.
"It may be possible to find excuses for each of these lapses - and others - but all of them, taken together, tell a different tale."
Scotland Yard will contact more than 50 people about the Partygate saga in the coming days, as it probes 12 events in Downing Street and Whitehall.
Mr Johnson is expected to be among those quizzed by police as he is believed to have attended six of the gatherings officers are investigating.
Downing Street failed to reject the allegation that staff had broken lockdown laws.
The Prime Minister's official spokesman said: "There's a Met investigation under way on these events, I simply wouldn't seek to comment."
Asked whether Mr Johnson's administration was "shifty", the PM's official spokesman said: "The Prime Minister wanted the full details of this to be looked into firstly by Sue Gray and he wants the Met investigation to be concluded as well.
"The Prime Minister initiated the investigation into these events."
