“This affair has done very considerable damage to the Prime Minister’s personal popularity,” he said in an interview with Times Radio.
“According to the polls, pretty consistently, three quarters of the public do not believe what the Prime Minister has been saying or his account of the case.”
At the next election, expected in 2024, many voters will see the Prime Minister as having delivered Brexit, having tried to deal with the cost-of-living crisis and having been “somewhat ahead of the game” in providing weapons to Ukraine, he explained, so these issues would all be factors which could sway their votes.
But on partygate he added: “There is no doubt that this particular issue has certainly led some voters to change their minds about the Prime Minister and whether or not they might be likely to vote for the Conservatives at the next election.”
He explained further: “Elections are not simply about the personal popularity of Prime Ministers.
“But certainly what is true is that once a political leader loses popularity, it’s very rare for them to regain it.
“John Major did his best after Black Wednesday in September 1992 when the Pound was forced out of the ERM [ European Exchange Rate Mechanism.]
“But the truth is to most voters that was just a signal that said this was a Government that was not in control of the Pound and it was never ever going to recover.
“The truth is that once it emerged that there were not any weapons of mass destruction that Saddam Hussein had that justified the war in Iraq, Tony Blair never recovered his popularity.
“And the question facing the Prime Minister is that people having come to the conclusion that actually there were what they at least regard as parties and not as work events...when they for example were not able to attend funerals, visit their relatives with dementia and all the rest of it, is whether or not that position is recoverable.
“The position the Government finds itself in on this issue, whereby it’s been repeatedly now for five months trying to persuade the public of its case, but frankly the polls not moving.
“The public really seem to have made their mind up about this back before Christmas and the polls have not moved substantially ever since.
“The instant polls we had yesterday painted a very similar picture of what we had before.
“We have had five months of Government ministers, Tory MPs trying to convince the public that actually what the Prime Minister did was reasonable and he was being honest.
“The public in five months have not been convinced.”
Many Tory MPs say their “mailbags” have not seen such a surge in angry letters and emails over partygate than during the storm over Dominic Cummings’ trip to Barnard Castle after the start of the first Covid lockdown in spring 2020.
But Sir John does make a comparison.
“What is the analogy..the analogy is Dominic Cummings’ infamous trip to Durham and then to Barnard Castle,” he said.
“The Government spent weeks trying to convince the public that it was reasonable...and they never succeeded in persuading the public of that case.
“The truth is the Government lost quite a lot of support in the wake of that and they have lost quite a lot of support now.”
As for the Prime Minister’s future, he continued: “Boris Johnson is a brilliant speaker, there are very few politicians who would have managed to work their way through the pressure that he has been under because of partygate in recent weeks.
“That’s why Tory MPs may still have faith in him.
“But you have to be aware that although that might enable you to survive inside the House of Commons, it’s so far at least not managed to persuade the jury in the court of public opinion.”