Claims that United Kingdom police investigating alleged government lockdown parties interviewed Prime Minister Boris Johnson under an official caution have been met with surprise.

The allegation that officers used the iconic phrase "anything you do say may be given in evidence", prompted Angela Rayner, deputy leader of the opposition Labour Party, to tell the UK television channel ITV News: "This is an embarrassment that, for the first time in UK history, we have a PM interviewed under police caution."

ITV News made the claim that Johnson and others were not questioned informally, as witnesses would have been, after seeing a leaked police email questionnaire that was sent to 88 government figures suspected of having attended illegal parties during the country's novel coronavirus lockdowns.

The questionnaire, which the broadcaster said has exactly the same weight as a face-to-face police interview, told recipients they "do not have to say anything", but notes failing "to mention, when questioned, something which you subsequently rely on in court may harm your defense".

The leaked email then asks recipients whether they have a "reasonable excuse" for attending social events during lockdowns, when all gatherings other than those necessary for work were prohibited.

The Guardian newspaper said it understands Johnson replied that he thought activities he attended were work-related, and therefore allowed.

Johnson's Downing Street office said he completed his questionnaire and returned it to the Metropolitan Police before the Feb 18 deadline.

His office said: "We have confirmed the prime minister has been contacted by the Metropolitan Police. We will not be commenting further while the investigation is ongoing."

The Times newspaper said Johnson consulted a lawyer at his own expense before completing the forms.

After finishing the criminal investigation, the police could impose fixed-penalty fines on people deemed to have broken rules. The fines could range from 100 pound ($136) to as much as 6,400 pounds.

Investigators have been probing 12 alleged social events that government politicians and officials may have attended. The Mirror newspaper said it understands Johnson may have attended six of them.

Senior civil servant Sue Gray has also looked into the allegations, on behalf of Parliament, and has prepared a report. However, that document has not yet been published in full because of concerns it could hinder the police investigation.

While Johnson has refused to say whether he would resign if he is found guilty of an offense or if he is criticized in Gray's report, many lawmakers from his ruling Conservative Party are understood to be closely watching the situation.

If 54 of the party's 360 lawmakers were to submit "letters of no confidence" in their leader, a leadership challenge would be triggered.

But some experts have said the open-ended questions in the police questionnaire suggest allegations about illegal lockdown parties are not being vigorously investigated.

The Mirror newspaper quoted former Metropolitan Police chief superintendent Dal Babu as saying the questions asked were "pretty bland".

He said a "lawyer would perhaps give you a 'get out of jail card' in response to all of those questions".