The Good Practices Guide for Computer Based Evidence is a guide published by the Association of Chief Police Officers of England, Wales, and Northern Ireland. It is the only current standard of practice for digital evidence. As this guide clearly states "Non compliance with this guide should not necessarily be considered as grounds to reject evidence.", but we believe that its principles are good ones, and we follow them.
Here are the four priciples they espouse:
Principle 1: No action taken by the police or their agents should change data held on a computer or other media which may subsequently be relied upon in Court.
Principle 2: In exceptional circumstances where a person finds it necessary to access original data held on a target computer that person must be competent to do so and to give evidence explaining the relevance and the implicaitons of their actions.
Principle 3: An audit trail or other record of all processes applied to computer based evidence should be created and preserved. An independent third party should be able to examine those processes and achieve the same result.
The officer in charge of the case is responsible for ensuring that the law and these principles are adhered to. This applies to the possession of and access to, information contained in a computer. They must be satisfied that anyone accessing the computer, or any use of a copying device, complies with these laws and principles.