An official investigation into government staff parties is to focus on three events that took place last year.
Downing Street parties on 27 November and 18 December, and at the education department on 10 December will be examined for Covid rule breaches.
Paymaster General Michael Ellis said any potential criminality uncovered would be reported to the police.
Labour called for the prime minister to resign if he is found to have misled MPs about the parties.
Shadow Cabinet Office minister Fleur Anderson questioned whether the investigation was “serious” and whether more parties needed to be included in it.
Boris Johnson announced on Wednesday that an inquiry would be carried out by top civil servant Simon Case, after a video from last year emerged of government staff joking about a party that took place on 18 December – a day on which more than 400 Covid deaths were reported.
Government spokeswoman Allegra Stratton resigned later on Wednesday over the video, obtained by ITV News, in which she appeared.
And Labour has called for Commons Leader Jacob Rees-Mogg to apologise after joking at an event on Monday that those attending would not be “investigated by the police in a year’s time”.
The Metropolitan Police has ruled out an investigation for the time being into the 18 December Downing Street gathering, citing an “absence of evidence” of any criminal wrongdoing.
On Wednesday, Mr Johnson said he had been “repeatedly assured” that “there was no party and that no Covid rules were broken” on that date – at which time London was under Covid rules banning people from meeting indoors.
But he said he had asked Mr Case to “establish all the facts” and that if rules had been broken those responsible would face disciplinary action.
The 10 December gathering took place during the same period of restrictions, while on 27 November England was in full lockdown.
Mr Ellis told the House of Commons on Thursday: “The primary purpose of the cabinet secretary’s investigation will be to establish swiftly a general understanding of the nature of the gatherings, including attendance, the setting and the purpose, and with reference to adherence to the guidance in place at the time.”
He added: “As with all internal investigations if, during the course of the work any evidence emerges of behaviour that is potentially a criminal offence, the matter will be referred to the police and the Cabinet Office’s work may be paused.”
Mr Ellis also said: “All ministers, special advisers and civil servants will be expected to cooperate with this investigation.”
But Mr Case will not look into reports of social gatherings in Downing Street on 13 November or a party at Conservative Party headquarters on 14 December, over which four members of staff have been disciplined.
