Released after weeks of speculation, it also found “leadership failings” in both Downing Street and the Cabinet Office.
The report also suggested that police are investigating 12 meetings that Sue Gray had seen as part of her investigation.
This includes one on November 13 to celebrate the exit of Dominic Cummings, a party that Boris Johnson had explicitly denied.
It came in a shortened report following a Met investigation into the parties at Number 10 and Whitehall, with police asking Ms Gray not to include the parties they are investigating.
The report said: “In the context of the pandemic, when the Government was asking citizens to accept far-reaching restrictions on their lives, some of the behavior surrounding these gatherings is difficult to justify.
“At least some of the meetings in question represent a serious breach not only of the high standards expected of those working at the heart of government, but also of the standards expected of the entire British population at the time.
“Sometimes it seems that very little thought was given to what was happening across the country when considering the appropriateness of some of these gatherings, the risks they presented to public health and how they might appear to the public.
“There were failures of leadership and judgment by different parts of No 10 and the Cabinet Office at different times. Some of the events should not have been allowed to take place. Other events should not have been allowed to unfold the way they did.”
Ms Gray explained that “it was not possible at this time to provide a meaningful report” establishing all that she discovered, due to the ongoing police investigation.
“As a result of the investigations by the Metropolitan Police, and in order not to harm the police investigative process, they have told me that it would only be appropriate to make a minimal reference to the concentrations on the dates they are investigating.
“Unfortunately, this necessarily means that I am extremely limited in what I can say about those events and it is not possible at this time to provide a meaningful report that establishes and analyzes the extensive factual information that I have been able to gather.”
The Gray report concluded: “The meetings within the scope of this investigation are spread over a 20-month period, a period that has been unique in recent times in terms of the complexity and breadth of the demands of public servants and, fact, the general public. .
“The entire country rose to the challenge. Ministers, special advisers and the Civil Service, of which I am proud to be a part, were a key and dedicated part of that national effort.
“However, as I have pointed out, a number of these meetings should not have been allowed to take place or develop in the way that they did. There is significant learning to be drawn from these events that needs to be addressed immediately across government. This does not need to wait for the police investigations to conclude.”
Scottish Labor leader Anas Sarwar called for the prime minister’s resignation.
He said: “The findings of the Gray report are clear: Boris Johnson has lied and is unfit to continue as UK Prime Minister.
“At a time of extreme difficulty for the people of the UK, it is clear that Boris Johnson has presided over an unacceptable workplace culture at number ten.
“Every second that Boris Johnson remains in office further degrades the office of prime minister.”
The Prime Minister will make a statement on the report on Monday afternoon.
In response to the report, Liberal Democrat leader Sir Ed Davey tweeted: “Everyone knows Boris Johnson broke the rules and lied to the country.
“It is time for Conservative MPs to do their patriotic duty, listen to their constituents and stand up for decency by firing Boris Johnson.
“He must go before he does more damage to our country.”
Shadow Foreign Secretary David Lammy tweeted: “During this crisis, our country needed a leader more than at any time since WWII.
“We didn’t get one. This report shows what we have known all along: the prime minister is a coward, a rule breaker and he needs to resign.”