Victoria's Criminal Bar Association says criminal cases could be jeopardised by police failing to follow proper protocols when signing affidavits.

A police officer told a Melbourne court yesterday that most officers do not take the required oath when signing affidavits used to obtain search warrants, or as evidence.

Victoria Police says it is seeking legal advice and has written to all inspectors to remind them about the proper process.

The association's chairman, Greg Lyon SC, has told ABC Local Radio that it is a breach of one of the most fundamental tenets of the criminal law.

"If this is widespread practice, the so-called affidavits that are going before the courts in relation to the execution of search warrants are not affidavits," he said.

"Magistrates have been granting search warrants in good faith, on the belief that they were receiving evidence on oath or affidavit, but they haven't been."

Mr Lyon says that could make the evidence invalid and could lead to appeals, even though the courts have discretion to allow illegally obtained evidence.

The secretary of the Police Association, Greg Davies, says most officers do not take an oath when signing an affidavit.

"It's an issue of training and it's an issue of reinforcing the standards that perhaps we were all imbued with several decades ago, but appear to have fallen by the wayside," he said.

Mr Davies says the oversight is going to be problematic at best.

"Clearly on the evidence that has been provided, there's suggestion that it will be a problem in many more than one instance."

Mr Lyon says it is an extraordinary breach.

"There are fundamental principles that really have to be re-instilled in the police, and this is one of them," he said.

"If they're not taught, they don't know, and the problem goes back to those in charge of teaching, and this has got to be a red flag for Victoria Police."

The acting president of the Law Institute of Victoria, Michael Holcroft, says it is a matter of great concern.

"It is simply beyond belief that Victoria Police had admitted during a serious criminal trial that they have not been following proper procedure with affidavits," he said.

"The effect is that evidence that has been put to the court may have been improperly obtained and could have been excluded."

Mr Holcroft says it is possible that appeals could result in cases where the only evidence upon which a conviction was based resulted from an improperly obtained search warrant.