Labour has branded Boris Johnson a “national embarrassment” after it was suggested he may have become the first British Prime Minister to be questioned under police caution over the partygate saga.

       A copy of the police questionnaire sent to Downing Street staff following partygate allegations was leaked to ITV news on Tuesday night.

       The Metropolitan Police is investigating 12 events, including as many as six which the Prime Minister is reported to have attended.

       Boris Johnson holds up a birthday cake – baked for him by school staff – during a visit to Bovingdon Primary Academy (Andrew Parsons/Downing Street)

       (PA Media)

       The investigations follow allegations of frequent and excessive drinking by Downing Street staff, to the extent where a wine fridge was purchased and staff were dispatched to local supermarkets to fill a suitcase with wine.

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       Mr Johnson has completed the questionnaire in relation to alleged parties, which the Met previously said had “formal legal status and must be answered truthfully”.

       The documents shared by ITV News appear to show a police caution advising those receiving the documents that they “do not have to say anything but it may harm your defence if you do not mention we questioned something which you later rely on in court”.

       Respondents are advised to “ensure the caution is read and understand prior to any answers to questions being provided”.

       According to ITV News, the document states that those sent the questionnaire are being provided with the “opportunity to cooperate with police in the form of a written statement under caution”.

       It says there are three ways to respond to the questionnaire: Remain silent and answer no questions, provide an answer to the written questions in the attached document or provide a prepared statement in your own words.

       ITV News said it asks around a dozen questions, including whether the person participated in a gathering on a specific date, what was the purpose of that participation, and whether or not the person interacted with anyone else at the gathering.

       The questionnaire asks for timings of the person’s attendance and how many others were present.

       ITV News said it also provides people with a chance to justify their actions, asking: “What, if any, lawful exception applied to the gathering and/or what reasonable excuse did you have for participating in the gathering?”

       In a major television interview on Sunday, Mr Johnson repeatedly refused to say whether he would quit if he broke the law.

       But Labour’s deputy leader Angela Rayner said: “It is frankly a national embarrassment that Boris Johnson is now the first Prime Minister in British history to be questioned under police caution.

       “How on earth can he stay on and claim to uphold British law and order after this?”

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       Tony Blair was questioned as a witness by police when prime minister over the cash for honours scandal, but never under caution.

       At the time, he said he would resign if he were interviewed under caution.