Scotland Yard has announced that it will start sending letters to Downing Street officials suspected of breaching lockdown rules after receiving a dossier of evidence from Sue Gray.
The Metropolitan Police said those alleged to have broken Covid-19 laws at government parties would be told to provide a “reasonable excuse” or be fined.
Those known to have attended events under investigation are expected to receive letters, raising the possibility Boris Johnson and his closest advisers will be contacted.
Mr Johnson will this week attempt to refocus on the day job, which will include a string of domestic policy announcements on Brexit and "levelling up", but this risks being undercut by the 'partygate' report.
Ms Gray is preparing to release her report within days - although Scotland Yard is insisting that she makes only “minimal reference” to the alleged lockdown-breaking parties it is now investigating.
Rather than wait until that inquiry is completed, The Telegraph understands Ms Gray has decided to publish her report, with changes in line with Metropolitan Police demands.
The developments leave Mr Johnson in limbo, unsure what will be contained in Ms Gray’s report or when it will be released, as Tory MPs weigh up whether to oust him.
It came on a day of finger-pointing and confusion about who is to blame for the delay in Ms Gray’s report.
On Friday morning, the Metropolitan Police had been accused of a “disproportionate” approach by insisting Ms Gray only reveals “minimal” information about the events it is now probing.
Legal experts and MPs had argued that there was no way any prosecutions launched from the inquiry could be prejudiced because they involved fines overseen by judges, not a jury.
On Friday night, a statement issued in the name of Commander Catherine Roper, who is overseeing the investigation, doubled down on the position, arguing it was “in order to protect the integrity of the police investigation”.
But the statement also revealed that relevant “material” had been delivered from the Cabinet Office, under which Ms Gray is conducting her investigation, to Scotland Yard on Friday.
One government source told The Telegraph the material could include hard evidence such as witness statements, photographs or text messages about the events being probed.
Part of the statement read: “In order to protect the integrity of the police investigation, as is appropriate in any case, and to be as fair as possible to those who are subject to it, the Met has asked for minimal reference to be made in the Cabinet Office report to the relevant events.
“This will only be necessary until these matters are concluded, and is to give detectives the most reliable picture of what happened at these events. We intend to complete our investigations promptly, fairly and proportionately.
“We have not delayed this report and the timing of its release is a matter for the Cabinet Office inquiry team.”
The statement also made clear the next steps for those accused of rule-breaking at the alleged events. The Telegraph understands eight gatherings are being investigated.
It read: “Individuals who are identified as having potentially breached these regulations will normally be contacted in writing, and invited to explain their actions including whether they feel they had a reasonable excuse.”
“Following this process, and where there is sufficient evidence that individuals have breached the regulations without reasonable excuse, officers will decide if enforcement action is appropriate.
“If the decision is to take enforcement action then a report will be sent to the ACRO Criminal Records Office which will issue the fixed penalty notice. Recipients can pay the fixed penalty and the matter will be considered closed.”
The Metropolitan Police earlier made clear it was only investigating claims of Covid rule-breaking, moving to end speculation that more serious alleged crimes had been uncovered by Ms Gray.
There was widespread criticism of the Metropolitan Police’s attempt to limit what Ms Gray could release.
Ken Macdonald, the former director of public prosecutions, said the move seems "disproportionate" in the face of "very powerful" public interest in the report's swift publication
Sir Roger Gale, the veteran Tory MP and one of the Conservatives to call for Mr Johnson's resignation, described it as a "farce" which could buy more time for the "lame duck" Prime Minister.