Scotland Yard announced it had launched a criminal investigation into the partygate allegations on January 25, but detectives have made slow progress in issuing the first fines.

It had been suggested that one of the reasons for the delay was that detetives, desperate to ensure all decisions were legally watertight, had been consulting Crown Prosecution Service (CPS) lawyers.

While there has been no formal involvement from the CPS so far, because the issuing of fixed penalty notices is not a matter for them, it is understood there have been discussions about the potential legal implications of the investigation.

Anyone who is issued with a fixed penalty notice will have 28 days to pay or can instead indicate they wish to contest the fine in court.

At that stage the police, with advice from the CPS, will need to decide whether they wish to continue with the case and bring it before the magistrates’ court.

A source said: “It is possible that all of those who are fined decide to fight the case in court. If that happens, it will present a huge headache for the police and prosecutors, who will need to decide what to do. 

“This scenario and various others will have been discussed before the first fines were actually issued.”

Neither the Foreign Office nor Downing Street issued a comment.