Boris Johnson has dismissed former Conservative prime minister Sir John Major's claim that the ongoing partygate scandal had damaged the UK's reputation on the international stage as "demonstrably untrue".
Earlier on Thursday, Sir John launched a stinging attack on the prime minister, accusing him of asking the public to "believe the unbelievable" over lockdown parties in Downing Street.
He added the UK's reputation overseas had "fallen because of our conduct".
In an apparent call-to-action for MPs to move against Mr Johnson, Sir John said: "Deliberate lies to parliament have been fatal to political careers - and must always be so".
"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."
Speaking later on Thursday, Mr Johnson told broadcasters he "will have plenty to say" about the police investigation into Downing Street parties "when the process is complete", adding that he is currently focussed on the situation at the Ukraine border.
Speaking at an event at the Institute for Government, Sir John accused the prime minister and his officials of breaking lockdown rules over parties held in Downing Street and across Whitehall - which are currently subject to a Metropolitan Police probe.
Speaking to reporters at a news briefing in Brussels earlier, Mr Johnson, who is coming under pressure to quit, said he was looking forward to Scotland Yard's investigation into 12 gatherings being completed.
Downing Street declined to reject Sir John's allegation, with the PM's official spokesperson telling reporters: "There's a Met investigation under way on these events, I simply wouldn't seek to comment."
Sir John, who was in office from 1990 to 1997, said: "At Number 10, 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."
Describing the behaviour of politicians in government as "corrosive", Sir John said: "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.
"When ministers respond to legitimate questions with pre-prepared soundbites, or half-truths, or misdirection, or wild exaggeration, then respect for government and politics dies a little more.
"Misleading replies to questions invite disillusion. Outright lies breed contempt."
It is not the first time Sir John has criticised Mr Johnson. He has been a vocal critic of Brexit, the prorogation of parliament and most recently the government's handling of the Owen Patterson lobbying scandal.
He previously called the government "politically corrupt" over its treatment of the House of Commons."
In his explosive intervention, Sir John continued: "The prime minister and our present government not only challenge the law, but also seem to believe that they, and they alone, need not obey the rules, traditions, conventions - call them what you will - of public life.
"The charge that there is one law for the government, and one for everyone else is politically deadly - and it has struck home."
Sir Malcolm Rifkind, who served as foreign secretary in Sir John's government, told Sky News later on Thursday that it was "a source of deep embarrassment that the prime minister's behaviour seems to have led to a police investigation".
"Our prime minister's authority both at home and overseas is substantially reduced by all the controversies of the last few weeks," he added.
"These matters clearly have not been resolved.
"So each time he goes to another capital city, you can bet your bottom dollar that what the newspapers and the media and the TV channels in these countries are reporting on are his troubles back home and not his views on Russia."
Opposition parties and some of Mr Johnson's own Conservative MPs have called for the prime minister to resign over the Downing Street 'partygate' scandal.
If 54 Conservative MPs submit letters of no confidence in the prime minister to the head of the 1922 group of backbench Tory MPs Sir Graham Brady, a vote on whether he should continue as leader will be held.
On Wednesday evening, Scotland Yard revealed officers working on their ongoing investigation into 12 lockdown-busting events held in Downing Street and across Whitehall - which has been named Operation Hillman - would this week start asking for the accounts of more than 50 attendees.
The news came just hours after a fresh photo emerged of Mr Johnson at a Christmas quiz in Number 10 alongside a bottle of alcohol, a packet of crisps and two people wearing tinsel and a Santa hat.
The Metropolitan Police is now rethinking its assessment of that event following the publication of the image.
At the time of the event, on 15 December 2020, London was under Tier 2 coronavirus restrictions, which banned any social mixing indoors between people from different households.
Scotland Yard said in a statement: "The Metropolitan Police service previously assessed this event and determined that on the basis of the evidence available at that time, it did not meet the threshold for criminal investigation.
"That assessment is now being reviewed."
The Met Police added that officers will contact people involved in the events by the end of the week and those contacted will initially fill out a questionnaire.
The Times cited policing sources as saying Mr Johnson himself was likely to be among those questioned.
The PM's official spokesman told reporters on Thursday that Mr Johnson has not been contacted by officers investigating partygate allegations.
"We would look to confirm contact of this sort as relates to the prime minister given the significant public interest," he added.
Asked about the matter at a news briefing in Brussels on Thursday, the prime minister said: "That process must be completed and I am looking forward to it being completed."
The developments come after the PM's new Commons leader claimed "real people" do not care about the partygate row.
Mark Spencer told BBC Radio Nottingham that "normal people" were more worried about the cost of living and the NHS backlog.
But Education Secretary Nadhim Zahawi contradicted this in an interview with Sky News.
Taking part in a Q&A with Kay Burley, he said: "There is no doubt that the stories around parties have cut through, people are rightly angry."
Prior to the Met Police probe, an investigation into gatherings held in Westminster was carried out by senior civil servant Sue Gray.
Ms Gray has handed more than 300 images and more than 500 pages of information to Scotland Yard.
She has yet to publish her full findings due to the ongoing police investigation.
There has been a recent exodus of senior advisers from Number 10 amid the partygate allegations.