Legal professional privilege protects the statements made and information disclosed by a client to their lawyer, from being compulsorily revealed to anyone else, including the court. Though described as a privilege, legal professional privilege is technically speaking a right, not a privilege. It has two distinct branches:
(a) Legal advice privilege, which protects the discussions between a lawyer and a client on any topic on which a client seeks advice in a relevant legal context. The rationale for legal advice privilege is that it is part of the administration of justice. A person must be able to speak freely to a lawyer without fear that that lawyer may be required to pass on what is said. Once privileged, a document is privileged for all time.
(b) Litigation privilege, which protects material such as reports and witness statements whose dominant purpose was preparation for a pending court case.
Both types were discussed in the case of Three Rivers District Council v Governor and Company of the Bank of England (No 6)  UKHL 48;  1 AC 610. Lord Scott of Foscote at para 38 cited with approval the definition of the scope of legal advice privilege given by Taylor LJ in the earlier case of Balabel v Air India  Ch 317, 330 where he said
“legal advice is not confined to telling the client the law; it must include advice as to what should prudently and sensibly be done in the relevant legal context”
Lord Scott went on:
“That there must be a ‘relevant legal context’ in order for the advice to attract legal professional privilege should not be in doubt. Taylor LJ said, at p 331, that ‘to extend privilege without limit to all solicitor and client communication upon matters within the ordinary business of a solicitor and referable to that relationship [would be] too wide’.”
Turning to litigation privilege, Lord Rodger of Earlsferry said at para 52:
“Litigation privilege relates to communications at the stage when litigation is pending or in contemplation. It is based on the idea that legal proceedings take the form of a contest in which each of the opposing parties assembles his own body of evidence and uses it to try to defeat the other, with the judge or jury determining the winner. In such a system each party should be free to prepare his case as fully as possible without the risk that his opponent will be able to recover the material generated by his preparations.”