Outrage erupted on social media after a video leak - in which senior No 10 aides joked about a Christmas party which Downing Street has denied ever happened.
The force said there was an “absence of evidence” and it is policy not to investigate retrospective breaches of coronavirus regulations.
Metropolitan Police said the video obtained by ITV “does not provide evidence of a breach” of health regulations and therefore the force would not commence an investigation.
In a statement, the force said: “The Metropolitan Police Service has received a significant amount of correspondence relating to allegations reported in the media that the Health Protection (Coronavirus, Restrictions) Regulations were breached at gatherings at No 10 Downing Street in November and December 2020.
“All this correspondence has been considered by detectives in detail, as well as footage published by ITV News. The correspondence and footage does not provide evidence of a breach of the Health Protection Regulations, but restates allegations made in the media. Based on the absence of evidence and in line with our policy not to investigate retrospective breaches of such Regulations, the Met will not commence an investigation at this time.
“The Met has had discussions with the Cabinet Office in relation to the investigation by the Cabinet Secretary. If any evidence is found as a result of that investigation, it will be passed to the Met for further consideration.”
In the footage obtained by ITV News, Allegra Stratton and adviser Ed Oldfield - along with other aides - were filmed joking about a “fictional” Downing Street party.
A laughing Ms Stratton said: “This fictional party was a business meeting … And it was not socially distanced.”
In the wake of the leak, a tearful Stratton resigned after a leaked video of her joking about a Downing Street party sparked outrage.
She fought back the tears as she announced she was stepping back from her role.
She said in a statement: “My remarks seemed to make light of the rules everyone. That was never my intention.”
“I will regret those remarks for the rest of my days and offer my profound apologies to all of you for them.”
She added: “Working in government is an immense privilege. I tried to do right by you all, to behave with civility and decency and up to the high standards you expect of Number 10.”