Former Client Conflicts and the Substantial Relationship Test
Loyalty to a former client is narrower than loyalty to a current one, but it does not expire. The question is whether the old matter and the new one are close enough that the lawyer would normally have learned something usable, and the answer rarely depends on what the lawyer says was learned.
The rule in short
A lawyer who formerly represented a client may not represent another person in the same or a substantially related matter where that person's interests are materially adverse to the former client, absent informed consent confirmed in writing. Matters are substantially related where they involve the same transaction or dispute, or where there is a substantial risk that confidential information normally obtained in the earlier work would materially advance the new client's position.
A representation ends, the file closes, and the duties do not all end with it. Two survive in full: the duty to protect information relating to the representation, and a duty of loyalty narrow enough to permit most new work while blocking a defined category of it. The rule that draws that line does most of the work in disqualification practice.
The three elements of the bar
The prohibition has three parts, and all three must be present. There must have been a representation, so the person must have been a client rather than an acquaintance, an opposing party the lawyer dealt with, or a witness who talked freely. The new matter must be the same as, or substantially related to, the old one. And the new client's interests must be materially adverse to the former client's.
Material adversity is a real limit. A firm that once advised a bank on a lease may take on a borrower in an unrelated transaction with that bank without offending the rule if the matters are unrelated, and may take on matters that touch the former client without opposing it. Adversity means the new work operates against the former client's interests, not merely that the former client would prefer the lawyer had declined.
How substantial relation is measured
The comment supplies the operative formulation, and it is framed around risk rather than proof. Matters are substantially related if they involve the same transaction or legal dispute, or if there is a substantial risk that confidential factual information as would normally have been obtained in the prior representation would materially advance the client's position in the subsequent matter.
Two features of that sentence matter. The information is the kind normally obtained, so the analysis proceeds by category rather than by an audit of what the lawyer actually saw. And it must materially advance the new client's position, so an abstract overlap in subject matter is not enough. A lawyer who defended one trucking accident does not thereby learn anything that advances a case about a different accident years later.
The inquiry is deliberately structured to avoid requiring the former client to reveal the confidences at stake in order to protect them. That is why courts generally reason from the scope of the earlier engagement rather than from testimony about what passed between lawyer and client, and why a lawyer's own assurance that nothing useful was learned carries limited weight.
The single most useful document in a disqualification fight is the original engagement letter. A letter that defined the work narrowly — one contract, one closing, one hearing — supports a narrow view of what was normally obtained. A letter describing general counsel services, or none at all, invites the opposite inference. The time to control this outcome is at intake.
Playbook knowledge and generally known information
Firms frequently argue that what they hold about a former client is only playbook knowledge: how the company approaches settlement, who signs off on what, how aggressive it is on discovery. Whether that suffices is genuinely contested. Some authorities treat general knowledge of policies and practices as insufficient on its own; others find disqualification where the playbook is detailed and directly usable against the former client in a comparable dispute.
A second qualification runs the other way. The rule permits use of information relating to a former representation where it has become generally known. That exception is narrower than it sounds. Information does not become generally known merely because it appears somewhere in a public record; it must be widely recognized by the relevant public or within the relevant industry. Disclosure of the information is treated more strictly than use.
| Relationship to the firm | Standard that applies | Screening to avoid imputation |
|---|---|---|
| Current client | Direct adversity or material limitation; adversity in unrelated matters counts | Not available; consent is the route |
| Former client | Same or substantially related matter, plus material adversity | Available in many states for a lateral hire, on conditions |
| Prospective client who did not retain | Information received that could be significantly harmful to that person | Expressly permitted by the model rule, with notice |
| Former government lawyer | Personal and substantial participation in the same matter | Expressly permitted, with screening and written notice to the agency |
| Person who consulted without seeking representation | No conflict rule engaged | Not applicable |
Consent, imputation and lateral movement
The bar is waivable. A former client may give informed consent confirmed in writing, and sophisticated clients regularly do, sometimes in the original engagement letter as a forward-looking waiver. What the writing must contain and when it must be delivered follow the same principles that govern consent to a conflict between current clients, with the same weakness: a waiver that describes no risks tends to be treated as no waiver at all.
Where consent is unavailable, the next question is whether the disability belongs to one lawyer or to the whole firm. A former client conflict is imputed to the firm as a general matter, but the model rule permits a firm to keep the matter where the disqualified lawyer arrived from another firm and is timely screened, subject to conditions and written notice. That mechanism, and the states that reject it, are the subject of the rules on imputation and screening a lateral hire.
What follows when the bar applies
A firm that cannot cure the conflict must decline the new matter or, if it has begun, leave it. Departure from a pending case is not a private decision: a tribunal's permission is generally required, and the departing firm still owes the client the protective steps that the rule on terminating a representation sets out, including surrender of papers and return of any unearned fee.
There is a second cost that is easy to overlook. Motions to disqualify are litigated, and the litigation itself exposes the earlier engagement to scrutiny. Billing narratives, memoranda and intake records become the evidence. Firms that keep thin records of what they did for a former client often find that the record does not help them, and the same documents that fail a conflicts defense can raise separate questions about how client funds and property were accounted for in the old engagement. The two inquiries are distinct, but they draw on the same files.
Points to carry away
- Duties to a former client are narrower than duties to a current client but do not lapse with time.
- The bar applies only where the new client's interests are materially adverse to the former client.
- Substantial relationship is judged by the risk that confidential information would normally have been obtained, not by proof that it was.
- A lawyer may not use information relating to the former representation to that client's disadvantage, except as the rules otherwise permit.
- Consent by the former client, confirmed in writing, removes the bar where the lawyer may properly proceed.
- States differ on how the test is phrased and on how playbook knowledge is treated.
Questions readers ask
Does the bar ever expire with the passage of time?
There is no fixed period after which a former client conflict disappears. The passage of time matters only through its effect on the substantial relationship analysis: information about a company's pricing formula or negotiating posture can become stale enough that it no longer offers any material advantage, and a court may say so. But the inquiry is about the continuing usefulness of the information, not about elapsed years. Information that remains current, such as the terms of a long-lived contract or a structural fact about a business, does not decay.
Is a prospective client who never retained the firm treated the same way?
No, and the difference is deliberate. Someone who consults a lawyer about forming a relationship is protected by a separate rule that is narrower in one respect and similar in another. The information disclosed during the consultation may not be used or revealed, and the lawyer is barred from a materially adverse matter only where the information received could be significantly harmful to that person. Firms that limit what they take in during an initial conversation preserve their ability to act later, and several states permit screening in this situation.
Who carries the burden on a motion to disqualify?
The party seeking disqualification generally has to establish the prior attorney-client relationship and enough about the subject matter of the prior work to show substantial relation. Once that is shown, courts commonly presume the sharing of confidences rather than requiring the former client to prove what was disclosed, since proving it would defeat the confidentiality the rule exists to protect. How strong that presumption is, and whether it can be rebutted, varies by jurisdiction. Motions of this kind are also judged against their potential use as a tactic.
Sources
- ABA Model Rules of Professional Conduct — Rule 1.9, Duties to Former ClientsThe text barring adverse representation in the same or a substantially related matter.
- ABA Model Rules — Comment on Rule 1.9Defines substantial relation and addresses generally known information and lapse of time.
- ABA Model Rules — Rule 1.18, Duties to a Prospective ClientThe narrower significantly harmful standard applied to consultations that never became representations.
- ABA Model Rules — Rule 1.6, Confidentiality of InformationThe underlying duty that survives the end of the representation.
- ABA Model Rules — Rule 1.10, Imputation of Conflicts of InterestHow a former client conflict travels with a lawyer who changes firms.
- District of Columbia Bar — Rules of Professional ConductAn adopted state-level text illustrating variation from the model in the former client provisions.
Justice Partners Journal is a publication, not a law firm. This article states general rules and cites its sources; it is not advice about any particular case, and the law differs by state and changes over time.
