Theresa May has expressed "anger" at the Downing Street party saga and warned "nobody is above the law".
In a letter to constituents, the Tory former Prime Minister took a thinly-veiled swipe at Boris Johnson, saying people working in Government "should conduct themselves with the highest of standards".
Mrs May broke her silence in a message to voters, published by her local paper, the Maidenhead Advertiser, where she warned "full accountability must follow" if rules have been broken.
It comes as the Whitehall inquiry into allegations of rule-breaking parties was plunged into chaos after the intervention of the Metropolitan Police.
Top civil servant Sue Gray was due to publish her partygate report this week but the bombshell news on Tuesday of a criminal investigation threw plans into doubt.
Scotland Yard has asked Ms Gray's team to only make "minimal reference" to the events it is investigating to avoid any prejudice to its inquiry.
Fears are mounting that this may force Ms Gray to delay publishing her report until the police probe is complete - or rush out a watered down version.
In a letter published before the Met Police announced its inquiry, Mrs May said: "I have said previously that it is vital that those who set the rules, follow the rules. Nobody is above the law.
"This is important for ensuring the necessary degree of trust between the public and Government.
"Like so many, I was angry to hear stories of those in Number 10, who are responsible for setting the coronavirus rules, not properly following the rules."
She said that "if there is evidence of deliberate or premeditated wrongdoing, I expect full accountability to follow".
Mrs May added: "All those working at the heart of Government should conduct themselves with the highest of standards which befits the work they do, and this applies as much to those working in No 10 as to other parts of Government."
The former PM has kept out of the spotlight since quitting No 10 in 2019 but she has been critical of Mr Johnson in the Commons over issues such as the cut to foreign aid spending.
Mr Johnson was a constant thorn in her side as she unsuccessfully battled to get a Brexit deal through Parliament.
His resignation from the Cabinet in protest at her Brexit strategy was seen as a major blow to her authority.
