If such explanations are not accepted, the police will then contact the ACRO Criminal Records Office, who will confirm the decision to issue the fine and will send notice by post.

If the person accused of breaching the Health Protection Act wishes to fight the allegation, they can opt to not pay the fine and then fight the case in the magistrates’ court.

In most cases, it will not be made public knowledge that someone has been given a fixed penalty notice, whereas court proceedings and convictions are public.

Investigating the Downing Street parties presents a challenge to the Metropolitan Police as most fines have been issued on the spot rather than retrospectively.

PM ‘could still be arrested’

Patrick Ormerod, criminal defence solicitor at Bindmans, said: “It is important to note that fixed penalty notices can only be given under the coronavirus regulations themselves.

“In some cases, for example the May 2020 garden party when the regulations had more loopholes in them than by December 2020, the Prime Minister might not have been personally guilty of an offence under the regulations but there may be a case to answer as an accessory or for encouraging or assisting breaches of the regulations by others under the Serious Crime Act 2007.”

“In those circumstances, a fine may not be available but he could still be arrested, if necessary, and prosecuted,” Mr Ormerod added.

Detectives working on the investigation have already begun work assessing the evidence provided by Sue Gray, but sources said they were “still some way off” taking any formal action against any of the accused.

While most people will simply receive notice of a fine through the post, it is understood that certain key figures including Boris Johnson and those accused of having organised allegedly illegal gatherings will be interviewed by police.