Boris Johnson is expected to tell MPs that he accepts full responsibility of failings, after the publishing of Sue Gray's report.
“I commissioned this report to set the record straight and allow us all to move on,” he is expected to tell MPs.
“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.
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.
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.”
Asked if she believes Mr Johnson will be in trouble following the report, she replied: “I am 100% behind the Prime Minister.”