The police will not say who is fined or confirm which events the fixed penalty notices relate to. However, Downing Street has said it would say if the prime minister is among those to receive a fine.
A sanction for breaking the law, fixed penalties need to be within 28 days or contested. If someone chooses to contest the fine, the police will then review the case and decide whether to withdraw the fine or take the matter to court.
It comes as a police investigation into 12 events across Westminster was launched in January, with a report by senior civil servant Sue Gray handed to the force.
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Prime Minister Boris Johnson is believed to have attended up to six of parties in question, with the police inquiry investigating if Covid regulations were broken at the events.
It is believed these 20 penalty punishments are only the first, and others are expected as more than 100 people were handed questionnaires by the Metropolitan Police as part of the investigation - named Operation Hillman.
The Met said it would contact people “believed to have taken part in the events in question to get their accounts”.
Sue Gray’s full report has yet to be published, with Mr Johnson facing cross-party demands to resign earlier this year over the parties as his leadership was plunged into crisis.
Detectives began interviewing witnesses last week ahead of issuing fixed penalty notices for breaches of Covid regulations.
The first referrals for fixed penalty notices (FPN) will be made to ACRO Criminal Records Office who will be responsible for issuing the FPNs to the individual following the referrals from the MPS.
In a statement, the Metropolitan Police said: “We are making every effort to progress this investigation at speed and have completed a number of assessments.
"However due to the significant amount of investigative material that remains to be assessed, further referrals may be made to ACRO if the evidential threshold is made.
"As it has for all fixed penalty notices issued during the pandemic, the MPS will follow the College of Policing Approved Professional Practice for Media Relations which states that “Identities of people dealt with by cautions, speeding fines and other fixed penalties – out-of-court disposals – should not be released or confirmed.”
"We will not confirm the number of referrals from each individual event subject to our investigation as providing a breakdown at this point may lead to identification of the individuals.”
Reports of the gatherings involving the Prime Minister created public anger and several Conservative MPs called for him to resign.
However, since the outbreak of the war in Ukraine, some – including Scottish Conservative leader Douglas Ross – have withdrawn their demand.
Johnson previously insisted that "guidelines were followed at all times" yet, he later apologised for attending a drinks party in the Downing Street garden in May 2020.
Kirsten Oswald MP, the SNP’s Deputy Westminster leader, said: “This damning development once again highlights the scale of rule-breaking at the heart of Boris Johnson’s corrupt government.“Boris Johnson should have resigned a long time ago over the boozy rule-breaking parties, but his ego and lack of dignity led him to desperately cling on.
“The reality is that the longer he stays in office the more lasting the damage will be.”