A report into parties held at British Prime Minister Boris Johnson's Downing Street office during coronavirus lockdowns said on Monday some gatherings represented a serious failing by those working at the heart of government.

"At least some of the gatherings in question represent a serious failure to observe not just 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," the report by senior civil servant Sue Gray said.

Johnson will deliver a statement to parliament on Monday after he received the report.

The prime minister, facing the gravest threat to his premiership over the alleged lockdown-busting parties at his residence and office at No. 10 Downing Street, has so far weathered growing calls to resign by asking angry lawmakers to wait for the report by senior civil servant Sue Gray.

London's Metropolitan Police said on Friday they had opened an investigation into some of the events to assess whether criminal offenses had been committed. They asked for the report to make only "minimal reference" to those events.

Weeks of media reports about more than a dozen gatherings – including a "bring your own booze" party in the Downing Street garden – have provoked widespread public anger, fueling the perception that the political elite failed to stick to the tough lockdown rules they set for the rest of the country.

Johnson has apologized for errors that were made and said he attended one garden party thinking it was a work event, but has rejected calls to quit.

Nevertheless, collapsing opinion poll ratings have unnerved lawmakers in his own party, raising the prospect that they could trigger a confidence vote in his leadership. Several have already publicly called for him to quit.

(With input from Reuters)