The police investigation into Downing Street parties is set to uncover evidence which has not yet been submitted to the Sue Gray inquiry, according to former No 10 staffers. The prime minister’s ex-chief adviser, Dominic Cummings, had previously warned that officials were deeply uncomfortable with handing over some evidence to the inquiry, believing they could face retribution for damaging information. But one senior Tory with knowledge of the inquiry said the announcement of a criminal inquiry on Tuesday made it a “different ballgame”, adding: “Officials who don’t tell Sue Gray the whole truth will not hold back from the cops.” Cressida Dick, the Metropolitan police commissioner, suggested Gray’s inquiry had already found what police believe to be evidence of wrongdoing. “I don’t anticipate any difficulty in obtaining the evidence that it is … necessary, proportionate and appropriate for us to obtain in order to get to the right conclusions,” Dick told members of the London assembly, as she confirmed police were investigating a number of alleged parties in Downing Street and Whitehall. Another senior source who had spoken to the Gray inquiry said there were HR obligations on current civil service staff to speak to the Cabinet Office probe but no such obligations for staff who have left. “There is a big dilemma for those who have something but haven’t been asked about it directly,” they said. “It’s one thing to be asked and not lie, it’s another to volunteer and then it becomes known you volunteered. “There’s a political reality to this investigation – depending on whether you think this prime minister can survive. Lots of people will be worried he will be wounded but not ousted.” Cummings said he had submitted written evidence to Gray by email with her consent. He said he would reply to further questions but would not be interviewed in person as he did not trust Boris Johnson not to leak his conversations. He has claimed Johnson was warned in advance that it was a breach of lockdown rules to hold a “bring your own booze” party on 20 May 2020, after an invitation was sent to about 100 Downing Street staff by Martin Reynolds, the prime minister’s principal private secretary. Cummings claimed he and another senior official warned in advance that the party was a mistake – but Johnson has said he believed the No 10 garden drinks constituted a work event. Cummings has said he will do an open question-and-answer session after the Gray report is published where he could make a series of new revelations, depending on the evidence in the report. The former adviser wrote on his blog that he believed officials would not hand over crucial evidence to the inquiry. “I know others are very worried about handing things to the Cabinet Office because they know the PM will see everything SG [Sue Gray] collects,” he said. “This inevitably means that evidence, including photos, is not given to her and instead will keep leaking after her report. To stress, this is a consequence of beliefs about the PM’s integrity, not SG’s.” Breaches of lockdown rules are dealt with via fixed penalty notices and are unlikely to count as a criminal offence unless resisted via the courts, meaning they would not have to be declared to an employer. Dick made clear during her evidence that the kind of offences being investigated could result in a fine.