As a result of today's announcement by the Metropolitan Police, Boris Johnson may now be interviewed under caution for an offence that carries a £100 fine. 

Why has the Met now decided to investigate?

Scotland Yard had said it would not investigate, but that changed after Whitehall ethics chief Sue Gray handed officers the outline of her findings on Sunday - and it revealed potentially criminal matters.

She is also believed to have spoken to police officers stationed at Downing Street.

They gave what were described as 'damning' witness statements about what had gone on during lockdown.

Met Commissioner Cressida Dick said her officers had made their own assessment that the material handed over met the bar for investigation including that there was actual evidence, 'not just someone saying something'.

Which events are the police examining?

Police have refused to provide details but said parties going back two years in Downing Street and Whitehall were being investigated.

Scotland Yard is thought to be focusing on eight of the 17 events linked to controversy.

Police may look at June 19, 2020, when Downing Street staff gathered in the Cabinet Room at Number 10 to share cake to mark Boris Johnson's (pictured) 56th birthday

They are thought to include May 20, 2020 'bring your own booze' event for 100 Downing Street staff in the No 10 garden. 

At the time, people could only meet one person from outside their household in a public space.

Boris Johnson has insisted he thought this was a work event.

Police may also look at June 19, 2020, when Downing Street staff gathered to share cake to mark Mr Johnson's 56th birthday in the Cabinet Room at Number 10. 

Indoor gatherings were against the law, although there were exemptions 'if reasonably necessary' for work purposes.

What are the penalties?

These are summary only offences, which do not carry a prison sentence.

They are punishable by fines, which are not recorded on the Police National Computer and Universitas Alma Ata Yogyakarta do not involve getting a criminal record. 

At the start of the pandemic, the fine was £100 for breaking the 'rule of six', which later doubled to £200.

Organisers of parties faced fines of up to £10,000. 

Anyone who does not pay the fine faces possible prosecution for breaching coronavirus regulations, with a higher fine and criminal record if convicted.

Will PM be interviewed under caution?

If police wanted to speak to Mr Johnson as a witness, he would not be interviewed under caution. 

But if the Prime Minister was suspected of an offence, he would probably be interviewed under caution, even if he voluntarily agreed to speak to officers.

Under the rules, a suspect will almost always be interviewed under caution if detectives wish to gather further evidence, elicit information revealing further lines of inquiry or give the suspect an opportunity to answer the allegations.

Scotland Yard had said it would not investigate, but that changed after Whitehall ethics chief Sue Gray (pictured) handed officers the outline of her findings on Sunday

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How long will it take and what will it cost?

Miss Dick refused to set a deadline yesterday. But the probe could take months given the number of parties being investigated, and the numbers of potential witnesses.

As for cost, the investigation into the cash for honours scandal - when Tony Blair was Prime Minister - took 16 months and cost £1.4million. 

It found that there was insufficient evidence to support a prosecution. Mr Blair was interviewed three times, but only as a witness.

Are there any legal exemptions?

The Covid regulations were brought in under the powers of the Public Health Act, but some lawyers have suggested the Act doesn't apply to Crown properties. 

However, his exemption has not been tested in law, and police have assessed the material provided to the Cabinet Office and decided to launch a criminal investigation.

Are there more serious offences?

If illegal gatherings were organised by officials in public office, it is possible police could investigate them for a separate more serious offence of misconduct in public office.

This can lead to a prison sentence