Boris Johnson has told ministers he is determined to give his full version of the Downing Street parties scandal in public, including his belief that there is a coordinated campaign to oust him. The prime minister has told cabinet allies he believes the police will clear him of any legal wrongdoing, the Guardian has been told. He is said to be frustrated that he is constrained by the criminal investigation and intends to set the record straight when it finishes. The prime minister has repeatedly told journalists in interviews he will have “something meaningful to say about this” when the Met investigation ends. It comes as Whitehall sources said multiple staffers had filled in Metropolitan police questionnaires stating only “no comment” in answer to questions posed as part of Operation Hillman, the inquiry into a dozen alleged events in breach of lockdown rules. There is growing nervousness among Downing Street staff that they will be handed fines while Johnson escapes without a fixed penalty notice (FPN). Some privately confessed to having little faith in the police investigation and said colleagues were likely to simply write “no comment” on the Metropolitan police questionnaire to avoid admitting guilt. Not all can afford expensive private legal advice as Johnson is expected to have received and are instead relying on pro bono help. The prime minister returned his Met questionnaire last week – one of more than 80 questionnaires reportedly set to staff members and aides. The Met has confirmed the number is more than 50. Johnson received external legal advice before submitting the questionnaire, a blank version of which was leaked on Tuesday. It revealed questions were asked under police caution, making Johnson the first prime minister to have been questioned under caution. A fresh batch of questionnaires were issued by Scotland Yard this week, suggesting Operation Hillman could continue into March. Whitehall insiders said they fear the Met investigation’s conclusions could come close to the May local elections, potentially in the “purdah” period. They warned this may further prolong the period before Sue Gray’s report into the parties was published in full. As well as asking the recipient to confirm their attendance at a specific event, the leaked questionnaire shows recipients were asked: “What was the purpose of your participation in that gathering?” and “Did you interact with, or undertake any activity with, other persons present at the gathering? If yes, please provide details.” Recipients were offered the opportunity to give a “lawful exception” or “reasonable excuse” for attending. The Met investigation into 12 social events, several of them attended by the prime minister, was triggered by evidence uncovered by Gray. Johnson’s allies are convinced he has a justification for attending events that took place at No 10, which is his home as well as his workplace. He initially denied reports that parties had been held in Downing Street, telling MPs: “All guidance was followed completely.” Many MPs and ministers have privately expressed the view that Johnson would probably face – and potentially lose – a vote of no confidence if he were to receive a fixed penalty notice for breaching the rules. But with the prime minister’s fate currently in limbo, one cabinet minister said on Tuesday that they believed enough doubt had been sowed among MPs to ensure the prime minister’s position was safe, particularly given the conflict in Ukraine. The minister said they believed there were now no more than 30 letters of no confidence submitted to the chair of the 1922 Committee, Graham Brady, with 54 needed to trigger a vote of MPs. A leading MP critical of Johnson said they believed rebels had lost their moment. “We have been poorly organised in the last few weeks, there’s no question,” they said. The MP, who has submitted a letter of no confidence, said they remained confident that at least 54 MPs would submit letters should Johnson be fined. “But Downing Street’s operation has succeeded in moving the goalposts,” they said. “Now colleagues are only prepared to move if there is a criminal offence. A month ago, it was if Sue Gray’s report was sufficiently damning. Now even some of my most critical colleagues are prepared to say in the WhatsApp group that it’s no longer an issue of concern for constituents.”