In the Court of Chancery, Lord Brougham said (page 100),

We are here to consider not the case which has frequently arisen in Courts of Equity, and more than once since I came into this Court, of a party called upon to produce his own communications with his professional advisers. How far he may be compelled to do so has, at different times, been a matter of controversy… the principle has been acted upon, that even the party himself cannot be compelled to disclose his own statements made to his counsel or solicitor in the suit pending, or with reference to that suit when in contemplation.

But the party has no general privilege or protection; he is bound to disclose all he knows, and believes, and thinks respecting his own case; and the authorities therefore are, that he must disclose also the cases he has laid before counsel for their opinion unconnected with the suit itself.

Here the question relates to the solicitor, who is called upon to produce the entries he had made in accounts, and letters received by him, and those written (chiefly to his town agent) by him, or by his direction, in his character or situation of confidential solicitor to the party; and I am of opinion that he cannot be compelled to disclose papers delivered, or communications made to him, or letters, or entries made by him in that capacity.

To compel a party himself to answer upon oath, even as to his belief or his thoughts, is one thing; nay, to compel him to disclose what he has written or spoken to others, not being his professional advisers, is competent to the party seeking the discovery; for such communications are not necessary to the conduct of judicial business, and the defence or prosecution of men’s rights by the aid of skilful persons.

To force from the party himself the production of communications made by him to professional men seems inconsistent with the possibility of an ignorant man safely resorting to professional advice, and can only be justified if the authority of decided cases warrants it.

But no authority sanctions the much wider violation of professional confidence, and in circumstances wholly different, which would be involved in compelling counsel or attorneys or solicitors to disclose matters committed to them in their professional capacity, and which, but for their employment as professional men, they would not have become possessed of. As regards them, it does not appear that the protection is qualified by any reference to proceedings pending or in contemplation.

If touching matters that come within the ordinary scope of professional employment, they receive a communication in their professional capacity, either from a client, or on his account, and for his benefit in the transaction of his business, or, which amounts to the same thing, if they commit to paper, in the course of their employment on his behalf, matters which they know only through their professional relation to the client, they are not only justified in withholding such matters, but bound to withhold them, and will not be compelled to disclose the information or produce the papers in any Court of law or equity, either as party or as witness.

If this protection were confined to cases where proceedings had commenced, the rule would exclude the most confidential, and it may be the most important of all communications—those made with a view of being prepared either for instituting or defending a suit, up to the instant, that the process of the Court issued. If it were confined to proceedings begun or in contemplation, then every communication would be unprotected which a party makes with a view to his general defence against attacks which he apprehends, although at the time no one may have resolved to assail him.
But were it allowed to extend over such communications, the protection would be insufficient, if it only included communications more or less connected with judicial proceedings: for a person oftentimes requires the aid of professional advice upon the subject of his rights and his liabilities, with no reference to any particular litigation, and without any other reference to litigation generally than all human affairs have, in so far as every transaction may, by possibility, become the subject of judicial inquiry….
The foundation of this rule is not difficult to discover. It is not (as has sometimes been said) on account of any particular importance which the law attributes to the business of legal professors, or any particular disposition to afford them protection, though certainly it may not be very easy to discover why a like privilege has. been refused to others, and especially to medical advisers.

But it is out of regard to the interests of justice, which cannot be upholden, and to the administration of justice, which cannot go on, without the aid of men skilled in jurisprudence, in the practice of the Courts, and in those matters affecting rights and obligations which form the subject of all judicial proceedings. If the privilege did not exist at all, every one would be thrown upon his own legal resources ; deprived of all professional assistance, a man would not venture to consult any skilful person, or would only dare to tell his counsellor half his case.

If the privilege were confined to communications connected with suits begun, or intended, or expected, or apprehended, no one could safely adopt such precautions as might eventually render any proceedings successful, or all proceedings superfluous.
From the terms in which I have stated the proposition, it is manifest that several cases may arise, which, though apparently they are exceptions, yet do in reality come within it. Thus the witness, or the Defendant treated as such, and called so to discover, must have learned the matter in question only as a solicitor or counsel, and in no other way : if therefore he were a party, and especially to a fraud (and the case may be put of his becoming informer after being engaged in a conspiracy), that is, if he were acting for himself, though he might also be employed for another, he would not be protected from disclosing; for in such a case his knowledge would not be acquired solely by his being employed professionally.

So if you examine the cases in which the protection has been refused you will find that they all range themselves within one or other of the following heads, which are deducible from the proposition and in strict consistency with its terms.

Those apparent exceptions are,
where the communication was made before the attorney was employed as such, or after his employment had ceased; or

where, though consulted by a friend because he was an attorney, yet he refused to act as such, and was therefore only applied to as a friend; or

where there could not be said, in any correctness of speech, to be a communication at all; as where, for instance, a fact, something that was done, became known to him, from his having been brought to a certain place by the circumstance of his being the attorney, but of which fact any other man, if there, would have been equally conusant (and even this has been held privileged in some of the cases); or

where the matter communicated was not in its nature private, and could in no sense be termed the subject of a confidential disclosure; or

where the thing disclosed had no reference to the professional employment, though disclosed while the relation of attorney and client subsisted; or

where the attorney made himself a subscribing witness, and thereby assumed another character for the occasion, and, adopting the duties which it imposes, became bound to give evidence of all that a subscribing witness can be required to prove.

In all such cases, it is plain that the attorney is not called upon to disclose matters which he can be said to have learned by communication with his client or on his client’s behalf, matters which were so committed to him in his capacity of attorney, and matters which in that capacity alone he had come to know…

Edited for ease of reading.