Police investigations into jury-nobbling allegations must be conducted with the "highest priority and urgency", leading judges have stressed.
Lord Chief Justice Lord Thomas and two other Court of Appeal judges in London sent out the message to all forces in England and Wales as they criticised the "unacceptably low standard" of an investigation carried out by Merseyside police into a jury-tampering claim made during a rape trial.
The judges directed that a copy of their written ruling - which sets out the duties of police when such allegations are made - should be sent to every chief constable.
Lord Thomas said on Friday that "as jury-nobbling/tampering undermines trial by jury", Parliament had given trial judges the express power to order the continuation of a case without a jury.
He added: "It is, in our view, implicit in those powers that it is the legal duty of the police to provide all the assistance a judge reasonably requires for the exercise of those powers.
"When, therefore, a judge hearing a trial requires the police to investigate an allegation of jury tampering in that trial, the investigation must be conducted under the close supervision of a senior officer of police who must personally provide regular reports to the judge as the investigation progresses."
He emphasised: "Moreover, it is essential that the investigation be conducted with the highest priority and urgency as the judge has to make a decision on whether to continue the trial with or without the jury as soon as is reasonably practicable, that is to say within a few days."
Lord Thomas, Sir Brian Leveson and Mr Justice Openshaw made their general observations on the duties of police in a ruling explaining their reasons for rejecting argument that a trial judge in Liverpool "wrongly concluded" there had been jury tampering in a case involving an allegation of rape, and had "therefore wrongly exercised his powers... to order the continuation of the trial without a jury".
Dismissing a challenge brought by the defendant, the appeal court ruled that the judge "had been correct in his decision", and that it was "entirely fair and in the interests of justice to continue the trial without a jury".
Lord Thomas said the judge was also "plainly right to proceed on the basis that it did not have to be shown that a defendant had to instigate jury tampering".
The legislation "only requires proof of jury tampering; it does not require proof of tampering by the defendant".
Lord Thomas said the objective of the legislation was to prevent the tampering - "it matters not that the defendant is not involved or not proved to be involved".
He ruled: "There was no reason for the judge to wait for the conclusion of the police inquiry. It added nothing material because, as is evident from the matters we have set out, the inquiry was of an unacceptably low standard."
He pointed out: "The Merseyside Constabulary should never have entrusted such a serious matter to such junior policemen, particularly given the well-known problems in relation to attempts to interfere with juries in Liverpool."