An official inquiry into the partygate scandal has said the "senior leadership" in Boris Johnson's Government must "bear responsibility" for the culture which led to coronavirus lockdown rules being broken.
The report by Sue Gray said the public would be "dismayed" by a series of breaches of coronavirus rules in No 10 and Westminster.
"The events that I investigated were attended by leaders in government. Many of these events should not have been allowed to happen," she said.
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The Metropolitan Police has issued 126 fines for rule breaches in No 10 and Whitehall, with the Prime Minister receiving a single fixed-penalty notice for his birthday party.
But senior civil servant Ms Gray condemned the wider culture that had been allowed to develop under Mr Johnson's leadership.
She said some of the more junior officials who attended parties "believed that their involvement in some of these events was permitted given the attendance of senior leaders".
"The senior leadership at the centre, both political and official, must bear responsibility for this culture," Ms Gray said.
She also said there were "multiple examples of a lack of respect and poor treatment of security and cleaning staff" during the events, which was "unacceptable".
"Many will be dismayed that behaviour of this kind took place on this scale at the heart of Government," she said.
"The public have a right to expect the very highest standards of behaviour in such places and clearly what happened fell well short of this."
Mr Johnson will apologise to MPs in a Commons statement later.
The report issued by Ms Gray includes a series of photos, with Mr Johnson pictured at the surprise birthday party in the Cabinet Room on June 19, 2020 for which he received a fine.
He is seen with Cabinet Secretary Simon Case and Chancellor Rishi Sunak, with sandwiches, juices and what appears to be Estrella lager - in one picture Mr Johnson is seen raising a can of the beer aloft.
Other photos include the previously-seen images of Mr Johnson raising a glass of wine at a leaving do for his former spin doctor Lee Cain on November 13, 2020.
The Prime Minister will make a Commons statement later on Wednesday, then hold a press conference to apologise directly to the public.
He will also address a meeting of Tory MPs who ultimately hold his political future in their hands.
In his Commons appearance, Mr Johnson is expected to say: "I commissioned this report to set the record straight and allow us all to move on.
"I accept full responsibility for my failings. I am humbled by the whole experience."
He will say he has learned the lessons from the situation.
