Downing Street staff have been warned not to "destroy any relevant information" regarding alleged Christmas parties that took place last year.
In an email seen by ITV News UK Editor Paul Brand, staff were warned any evidence of a criminal offence will be passed to the police, which could pause the internal investigation at that stage.
Staff were also told some of them would be summoned in the coming days to give evidence to the ongoing investigation.
ITV News UK Editor Paul Brand explains the contents of an email sent to Downing Street staff relating to the alleged Christmas parties
The message also notes emails are automatically deleted after 90 days - as is standard security policy - meaning anything relating to a year ago will have to be recovered.
The email was sent from Boris Johnson's Principal Private Secretary Martin Reynolds and offers staff HR support if they need it.
Mr Reynolds appears to be assisting with the running of Cabinet Secretary Simon Case's investigation and also works at the heart of government, which raises questions about how objective the internal investigation can really be.
The prime minister recently ordered an inquiry into alleged rule-breaking in Downing Street after ITV News released a video showing some of his senior advisers laughing about a party which allegedly took place on December 18 last year, while London was under Tier 3 restrictions.Mr Johnson confirmed that the investigation, being carried out by Cabinet Secretary Simon Case, will look into a quiz - after a picture emerged of him allegedly hosting it on December 15 - along with accusations that rules were broken on December 10 at a festive drinks party held at the Department for Education.
But when asked about the reports, the PM said: "I can tell you that I certainly broke no rules."
Downing Street has confirmed Mr Johnson “briefly” attended the “virtual” quiz at the tail end of last year.
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An image published by the Sunday Mirror shows the Prime Minister flanked by colleagues, one of whom is draped in tinsel and another wearing a Santa hat, in the No 10 library.
Tier 2 restrictions that London was under on December 15 stated that there could be no mixing of households indoors, apart from support bubbles, and a maximum of six people outside.
Official guidance set out that there should not be work lunches or parties “where that is a primarily social activity” and was not exempted for “work purposes.”
The Metropolitan Police said they do not intend to investigate claims of Christmas parties in Downing Street at this time, despite the photographic evidence published by the Mirror.