Boris Johnson has failed to prove doesn't consider himself above the rules, the chief of the standards watchdog has warned.
Lord Evans, chairman of the powerful Committee on Standards in Public Life, said the Partygate scandal left “questions” on whether standards are being upheld in Government.
The former MI5 chief also blasted the Prime Minister's attempt to water down the ministerial code, which governs the conduct of top politicians.
Mr Johnson came under fire for issuing a new version of the code, which would allow ministers who break the rules to avoid resigning.
Traditionally, politicians who breach the code are obliged to quit.
Mr Johnson also blocked a bid to give his ethics chief power to investigate him.
Lord Evans told the Today programme: "I think you’ve got to raise questions when you see the outcome of the police investigations and the Sue Gray report, and one or two of the other issues that have come up – I was outspoken myself in regard to the Owen Paterson business.
"So, there has been a lot of public disquiet about standards over the last six months.
"It’s one of those things that comes up from time to time and it’s really important to reassure people that we want to continue to maintain decent standards in this country."
In an online blog, he said the PM's reforms were "highly unsatisfactory" and "suspicion about the way in which the Ministerial Code is administered will linger" if his independent adviser isn't given greater powers.
It comes as the PM's ethics chief Lord Geidt said his Partygate fine may have broken the ministerial code.
In a damning report slipped out by No10 ahead of the Jubilee, Lord Geidt said there were "legitimate questions" over whether the PM's birthday party penalty broke ethics rules and demanded Mr Johnson explain himself.
He also criticised Mr Johnson for refusing his demand to start his own investigations without the PM's permission.
Deputy Labour leader Angela Rayner said the PM was trying to "rig the rules" as he faces further questions about his conduct.
She said: " Boris Johnson has shown himself unfit to uphold high standards in public life and is lowering the bar each and every day he clings to office, trying to rig the rules as he faces investigation for deliberately misleading Parliament.
"The Chair of the Committee on Standards in Public Life joins the Prime Minister’s own ethics adviser in criticising his cherry-picking of changes to the Ministerial Code and ethical scrutiny of Government. It’s time to stop the rot."
It comes as the embattled PM received a Jubilee reprieve from Tories plotting to oust him in from No10 in the wake of Sue Gray's Partygate report.
Speculation is mounting that a no confidence vote could be triggered as early as next week - but no movement is expected until after the Queen's Platinum Jubilee.
Loyalist Priti Patel told colleagues not to overshadow the Jubilee by trying to oust Mr Johnson and urged those trying to get him to quit to "forget it".
The Home Secretary told the Daily Mail: "This isn’t about a parade (of leadership candidates) or a contest of letters. We need to concentrate on doing our jobs."
On a visit to the Mall ahead of the celebrations, she said: "Events like this are about celebrating our country, our nation, our values and our monarchy.
"This is about Her Majesty the Queen – I think that’s where all the focus should be."
Top Tory Jacob Rees-Mogg said it would be "exceptionally silly" to try to oust the PM.
"He's an enormous electoral asset," he told TalkTV.
"It would be the most divisive thing that the party could do. It's an exceptionally silly thing to want to try and open the door to Sir Keir Starmer - assuming he manages to survive."
It comes amid reports senior Tories had urged Mr Johnson to call a confidence vote in himself ahead of two crunch by-elections on June 23 to "force the issue".
