The job will be made more complicated by the fact that the rules – which were far from straightforward – changed regularly over time as the pandemic progressed.

The majority of gatherings also occurred in places where the accused worked, so are not as clear cut as when a member of the public was caught, for example, at an unlicensed music event.

Once the Met have determined which gatherings were illegal under the Health Protection (Coronavirus) Regulations, they will then need to identify both the organisers and the attendees.

This should be relatively straightforward as there will be security swipe card and other records available.

Culprits will be fined

Any person who is identified as having been at one of the illegal gatherings will then be contacted in writing or by email by the police to inform them they are being issued with a fixed penalty notice.

If they wish to challenge the decision they can respond to the police and offer an explanation as to why they should not be fined.

This could include suggestions that it was a case of mistaken identity or that they were working, rather than participating in any lockdown-breaking parties.

At this stage they might be interviewed by the police in order to give their version of events in detail.

If their explanation is 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 lockdown rules wishes to fight the allegation, they can opt to not pay the fine and then take the case in the magistrates’ court.

What evidence does the police have?

The evidence was gathered by Sue Gray during her investigation and was handed over to Scotland Yard.

Police are expected to take weeks to go through all the evidence before starting the process of contacting those who they wish to fine.

The Met has a large cache of evidence including more than 500 documents and thanks to the existence of camera phones, more than 300 images.

As well as the documentary and photo evidence, the police could also rely on a number of admissions and apologies that have been issued by key figures in recent weeks.

There is also a wealth of potential evidence stored on Downing St security and CCTV networks as well as testimony from police officers stationed in the area at the time.

Who is being investigated or questioned and why?

Is it not clear how many people are being investigated by Scotland Yard but there are reports that dozens of people attended the 12 gatherings now under the spotlight.

Anyone who was at an illegal gathering but cannot provide a reasonable excuse is likely to be fined.

If the police can establish who was responsible for organising illegal gatherings, they could be in line for much higher fixed penalty notices up to £10,000.

Among those who are likely to feature in any police investigation are Martin Reynolds, who sent out the invitation for the infamous ‘bring your own booze’ party; Carrie Johnson, who has been accused of organising two events and Simon Case the top civil servant who had to recuse himself from the inquiry when it emerged an event had been held in his own office.

James Slack, the former government communications chief, who is now at the Sun newspaper, has apologised for the anger and hurt that was caused when it emerged he had held a leaving party the night before the Duke of Edinburgh’s funeral.

Finally the Prime Minister, who has admitted attending the Downing Street garden party on May 20 2020, is likely to be investigated even though he has insisted he did not realise it was a party and thought it was a work event.

How long is the inquiry likely to take?

Scotland Yard has refused to put a timescale on the investigation and there is no real precedent for a retrospective probe like this.

Sources within the Met have suggested it could be weeks and possibly even months before people begin to receive notifications that they will be getting fixed penalty notices.

The Met is also yet to decide how many officers to put on the investigation. A final decision will be made once all the evidence has been assessed.

But the Met has insisted the investigation must remain proportionate and senior officers will be keen to avoid criticism that they are using valuable resources on an offence that carries a maximum penalty of a fine, at a time when teenage homicides are at a record level.