Per Lord Neuberger in FHR European Ventures LLP v Cedar Capital Partners LLC  UKSC 45 at paras 5-7:
5. The following three principles are not in doubt, and they are taken from the classic summary of the law in the judgment of Millett LJ in Bristol and West Building Society v Mothew  Ch 1, 18. First, an agent owes a fiduciary duty to his principal because he is "someone who has undertaken to act for or on behalf of [his principal] in a particular matter in circumstances which give rise to a relationship of trust and confidence". Secondly, as a result, an agent "must not make a profit out of his trust" and "must not place himself in a position in which his duty and his interest may conflict" - and, as Lord Upjohn pointed out in Boardman v Phipps  2 AC 46, 123, the former proposition is "part of the [latter] wider rule". Thirdly, "[a] fiduciary who acts for two principals with potentially conflicting interests without the informed consent of both is in breach of the obligation of undivided loyalty; he puts himself in a position where his duty to one principal may conflict with his duty to the other". Because of the importance which equity attaches to fiduciary duties, such "informed consent" is only effective if it is given after "full disclosure", to quote Sir George Jessel MR in Dunne v English (1874) LR 18 Eq 524, 533.
6. Another well established principle, which applies where an agent receives a benefit in breach of his fiduciary duty, is that the agent is obliged to account to the principal for such a benefit, and to pay, in effect, a sum equal to the profit by way of equitable compensation. The law on this topic was clearly stated in Regal (Hastings) Ltd v Gulliver (Note) (1942)  2 AC 134, 144-145, by Lord Russell, where he said this:
"The rule of equity which insists on those, who by use of a fiduciary position make a profit, being liable to account for that profit, in no way depends on fraud, or absence of bona fides; or upon such questions or considerations as whether the profit would or should otherwise have gone to the plaintiff, or whether the profiteer was under a duty to obtain the source of the profit for the plaintiff, or whether he took a risk or acted as he did for the benefit of the plaintiff, or whether the plaintiff has in fact been damaged or benefited by his action. The liability arises from the mere fact of a profit having, in the stated circumstances, been made."
7. The principal’s right to seek an account undoubtedly gives him a right to equitable compensation in respect of the bribe or secret commission, which is the quantum of that bribe or commission (subject to any permissible deduction in favour of the agent – eg for expenses incurred). That is because where an agent acquires a benefit in breach of his fiduciary duty, the relief accorded by equity is, again to quote Millett LJ in Mothew at p 18, "primarily restitutionary or restorative rather than compensatory". The agent’s duty to account for the bribe or secret commission represents a personal remedy for the principal against the agent. However, the centrally relevant point for present purposes is that, at least in some cases where an agent acquires a benefit which came to his notice as a result of his fiduciary position, or pursuant to an opportunity which results from his fiduciary position, the equitable rule ("the Rule") is that he is to be treated as having acquired the benefit on behalf of his principal, so that it is beneficially owned by the principal. In such cases, the principal has a proprietary remedy in addition to his personal remedy against the agent, and the principal can elect between the two remedies.