Boris Johnson has become the first British prime minister to be interviewed under police caution.
A copy of the Metropolitan Police questionnaire sent to Johnson and 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.
Johnson returned his completed questionnaire to the police on Friday night.
According to the leaked copy, recipients were told 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”.
The document goes on to say that those sent the questionnaire are being provided with the “opportunity to cooperate with police in the form of a written statement under caution”.
Among the questions posed are “What, if any, lawful exception applied to the gathering and/or what reasonable excuse did you have for participating in the gathering?”
When Tony Blair was prime minister during the cash for honours scandal, he was interviewed by police as a witness.
He let it be known to aides that if he were interviewed under caution, he would have to resign.
Labour 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?”
Downing Street has been approached for comment.