Simon Case, the head of the civil service, has emerged as one of those to have been sent a questionnaire about the No 10 lockdown parties, amid continuing speculation that senior government figures will be interviewed by police. It is understood Case was sent a quiz by the Metropolitan police in the same round of questionnaires as other senior government figures, including the prime minister and chancellor. The Met said earlier this week that it intended to start questioning witnesses, prompting speculation that Johnson himself could be spoken to by officers. No 10 last confirmed on Tuesday that Boris Johnson had not been interviewed at that point. A No 10 spokesperson said on Friday: “We don’t have anything further to add.” However, it is understood his allies believe he will not be questioned because the police said they wanted to speak to witnesses, rather than those under investigation. Asked whether Case had been interviewed by police, a Cabinet Office spokesperson said: “It would not be appropriate to comment on speculation. The cabinet secretary and the prime minister are fully focused on the vital work this government is doing on the illegal invasion of Ukraine.” The police statement – issued two months after inquiries began – suggested officers have not yet found breaches that meet the evidentiary threshold for fixed-penalty notices to be issued without further interviews. No FPNs had been issued so far, the force said. The update suggested the investigation may be some way off being concluded. The Met said it “involves a significant amount of investigative material; the serving of over 100 questionnaires and the need to individually assess every response”. It added: “The offences under consideration comprise a number of elements and the legislation itself changed between the event dates. We are progressing the investigation as quickly as possible. “As yet, we have not made any referrals to the Acro criminal records office for the issuing of fixed-penalty notices. However, every questionnaire response is being assessed alongside all available evidence, and should this reach the evidential threshold, then referrals will be made.” Ministers are publicly expressing confidence that Johnson will not be issued with a penalty by police. Oliver Dowden, the Tory co-chair, told LBC: “The prime minister is actually absolutely resolutely clear that he is not going to be subject to a fixed-penalty notice because he is confident that he has not broken the law.” The announcement that the Met were now seeking interviews with those involved prompted incredulity from senior Whitehall sources. “The police have had all the evidence they need that rules were broken for months,” one said. It comes two months after police began examining material from the Whitehall inquiry by the senior civil servant Sue Gray, meaning it is now unlikely a verdict on Johnson will be reached before the May local elections. Scotland Yard obtained more than 300 photographs and 500 pages of documents showing what Gray’s inquiry believes to be potential rule-breaking. Images include photos taken at parties and those taken from security-system cameras showing when people entered and exited buildings.