Scotland Yard has questioned attendees at Downing Street lockdown parties under caution, a leaked document has revealed.

It raises the prospect that Boris Johnson has become the first Prime Minister to be questioned under caution by the police.

The document appears to be a copy of the questionnaire issued by the Metropolitan Police to those accused of attending illegal gatherings.

It contains the standard Police and Criminal Evidence Act caution which states: “You do not have to say anything, but it may harm your defence if you do not mention when questioned something you later rely on in court. Anything you do say may be given in evidence.”

On Tuesday night, Angela Rayner, Labour’s deputy leader, described it as an “embarrassment” for the Government and Mr Johnson personally.

She said: “This is an embarrassment that for the first time in UK history we have a PM interviewed under police caution.”