Boris Johnson must “bear responsibility” for the culture in Downing Street during the pandemic that allowed repeated breaking of Covid rules, senior civil servant Sue Gray concluded in her long-awaited verdict on the partygate scandal.

Gray’s 37-page report — plus photographs — is an indictment of lawbreaking and drinking in 10 Downing Street in the middle of a national crisis, when the rest of the country was observing Covid lockdowns.

Johnson, who was fined by the police for his participation at one party, has come under renewed criticism from Conservative MPs for allowing ill-discipline and illicit behaviour to take place under his own roof.

The leadership of the civil service is also strongly criticised by Gray, but Simon Case, the head of the civil service, is expected to stay in his position, according to senior government figures.

Johnson will be relieved that the report is not as personally critical of him as some had expected, although Gray said the problems exposed in Downing Street came from the top.

Gray’s report, which includes nine photographs of Johnson at various events, concluded that “many of these events should not have been allowed to happen”.

She added: “It is also the case that some of the more junior civil servants believed that their involvement in some of these events was permitted given the attendance of senior leaders.”

Pointing the finger at both Johnson and Case, she concluded: “The senior leadership at the centre, both political and official, must bear responsibility for this culture.”

Gray concluded in her report: “Many will be dismayed that behaviour of this kind took place on this scale at the heart of government. 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.”

However, she said: “It is my firm belief . . . that these events did not reflect the prevailing culture in government and the civil service at the time.

“Many thousands of people up and down the country worked tirelessly to deliver in unprecedented times. I remain immensely proud to be a civil servant and of the work of the service and the wider public sector during the pandemic.”

Johnson will give a statement on the lessons learned from Gray inquiry at 12.30pm and is braced for renewed criticism of his leadership from Tory MPs. At 5pm he will address his party behind closed doors.

He is expected to tell MPs: “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. We have learned our lesson.”

Before Gray’s report was published, most Conservative MPs had concluded that Johnson would survive the partygate affair, albeit with his reputation badly tarnished in the eyes of many voters.

However, the details contained in the report on the culture of parties and drinking at the heart of government has disturbed many Tory MPs; earlier this month the party lost almost 500 seats in local council elections.

Johnson’s allies are confident the prime minister will not face a no-confidence motion — 54 Tory MPs are required to trigger such a contest — in spite of the latest revelations.

The prime minister hopes to move quickly on from the partygate scandal; chancellor Rishi Sunak is expected to bring forward a package of measures to alleviate rising energy bills on Thursday.

Gray released an interim report in January. That criticised the leadership within Number 10, but the full detailed report was put on hold until the Metropolitan Police concluded its own inquiries.

The police investigation ended last week, with 126 fines given out over events on eight dates. Johnson was fined for attending an impromptu birthday party in the cabinet room.