The former prime minister's intervention is significant as she has noticeably said very little during the development of the Partygate scandal.
Johnson is coming under increasing pressure to resign following a string of alleged parties, gatherings and 'work events' in Number 10 during lockdown while the rest of the country was told to abide by strict coronavirus laws.
Calls for Johnson's resignation have grown both within and outside of his party, with senior Tory MPs such as Andrew Bridgen and David Davis calling for to go. Davis dramatically told the prime minister "in the name of God, go" during a rowdy session of prime minister's questions last week.
Speaking to her local newspaper, The Maidenhead Advertiser, May said earlier this week that "nobody was above the law" and that, like members of the public, she was "angry" to hear of the reported parties in Number 10 when they set the rules.
"When the report's findings are published, if there is evidence of deliberate or premeditated wrongdoing, I expect full accountability to follow," she said.
"All those working at the heart of government should conduct themselves with the highest standards which befits the world they do, and this applies as much to those working in Number 10 as other parts of government."
What "full accountability" would look like is up for debate, with some calling for Johnson's resignation if laws were broken.
Watch: Partygate: Timing of Sue Gray report publication thrown into doubt after Metropolitan Police request 'minimal reference' to events it is investigating
Douglas Ross, leader of the Scottish Conservative, called for the prime minister's resignation earlier this month after he admitted attending one of the Downing Street parties.
The prime minister himself ordered the inquiry after public and political outrage over suspected rule breaking on Downing Street and Sue Gray, the senior civil servant leading the investigation, was set to release her findings this week.
However exactly how, and when, the findings will be published has been thrown into question following a significant intervention from the Met police, who have requested elements of Gray's report should be held back while they conduct their own investigations.
"For the events the Met is investigating, we asked for minimal reference to be made in the Cabinet Office report," the Met said on Friday.
"The Met did not ask for any limitations on other events in the report, or for the report to be delayed, but we have had ongoing contact with the Cabinet Office, including on the content of the report, to avoid any prejudice to our investigation."
Earlier this month, senior Conservative MP Sir Roger Gale told Yahoo News UK that Theresa May would "absolutely not" have operated Number 10 in the way that Johnson has.
"If Theresa had a party, she had a party that was legal and formal and above board - with proper caterers," he said.
A potential delay to the release of the report is likely to trigger frustration among Conservative MPs who were reportedly waiting for the report before deciding whether to lodge a letter of no confidence in the prime minister.
There are some arguing that a delay to the release of the report, and aspects of Gray's inquiry being held, because of a police investigation will work in Johnson's favour.
However, a senior Conservative source told Yahoo News UK on Wednesday the prime minister wanted the matter to be resolved and dealt with as soon as possible so everyone can "move on" from it.