Can You Sue an Expert Witness in Ontario? What the Law Says

Can you sue an expert witness who gave evidence against you in Ontario?

No — Ontario law gives expert witnesses absolute immunity from civil lawsuits for anything they say or write as part of court proceedings. In O’Regan v. Harman and Romain, 2026 ONSC 4066 (CanLII), the Superior Court of Justice struck a claim that tried to hold an expert witness liable in negligence, misrepresentation, and contract. The court found the claims were barred by witness immunity and that the alleged damages were too speculative to survive even a generous reading of the pleading.

What is witness immunity and does it cover expert reports?

Witness immunity — sometimes called absolute privilege — is a rule that protects anyone who gives evidence in legal proceedings from being sued for what they said or wrote in that role. Yes, it covers expert reports. Courts have consistently held that the protection extends to written reports prepared for use in litigation, not just oral testimony delivered from the witness stand.

The logic is practical: if experts could be dragged into separate lawsuits every time the party they testified against lost money, qualified professionals would refuse to take on litigation work. The justice system depends on candid, independent expert opinion, and absolute privilege is what makes that possible.

Does it matter if the expert was wrong or acted carelessly?

No — that is precisely what makes this privilege ‘absolute.’ Even if an expert’s report contained errors, even if the expert was careless, and even if the evidence contributed to an unfavourable outcome for the other side, a civil lawsuit against that expert will not succeed. Ontario courts have applied this principle consistently, following decisions such as Paul v. Sasso and Due v. Collings.

In the O’Regan case, the claims were framed in three different ways — negligence, misrepresentation, and breach of contract — but the court looked past the labels to the substance. Because all three complaints were really about the expert’s litigation work, all three were struck.

What does a court look for on a motion to strike a claim?

On a motion to strike under Rule 21.01(1)(b) of the Rules of Civil Procedure, the court asks whether it is plain and obvious that the claim discloses no reasonable cause of action. The pleading is read generously, giving the plaintiff every reasonable benefit of the doubt. Even under that generous standard, the claim in O’Regan could not survive.

The court also found that the damages alleged were speculative. Because no court had yet determined the outcome of the underlying action the expert had testified in, any claim for financial loss was conjecture — there were no material facts to support a damages award, whether compensatory or punitive.

Can you fix a fatally flawed claim by amending it?

Generally, courts allow amendments liberally under Rule 26.01, but they will refuse leave in the clearest of cases. Here, the court denied leave to amend because no rewording could overcome two fundamental problems: absolute witness immunity is an unanswerable bar, and the speculative damages could not be cured by adding more particulars when the underlying loss had not yet been established.

This is an important reminder that some legal defects cannot be papered over. If the core of your claim is barred by law, amending the pleading will not save it.

What options do you actually have if an expert harmed your case?

Several avenues remain open, even though a civil lawsuit against the expert is not one of them. You can cross-examine the expert aggressively at trial, retain your own competing expert, or bring a motion to exclude the evidence if it does not meet the threshold for admissibility. If you believe the expert committed perjury or professional misconduct, you can report them to their regulatory body or raise the issue with the court.

If the real concern is that the outcome of your case was unjust, the more productive question is whether errors were made in how your own case was prepared or argued. Our Ontario employment lawyers regularly advise clients who are navigating complex litigation involving expert evidence.

Practical takeaways for litigants facing expert evidence

• Challenge the expert during the proceeding, not after. Cross-examination and competing experts are your tools — a post-proceeding lawsuit is not.
• Watch for speculative damages claims. If your loss depends on the outcome of another proceeding that has not yet concluded, a court may strike your claim as premature.
• Changing the legal label does not change the substance. Courts look at what a claim is really about, not what it is called. Framing an immunity-barred claim as ‘contract’ instead of ‘negligence’ will not help.
• Amendments have limits. If a claim is barred by an absolute rule of law, no amendment can fix it — courts will deny leave rather than allow futile pleading exercises.
• Act early. If you have concerns about expert evidence being used against you, raise them with your lawyer before or during the proceeding, not years later in a fresh lawsuit.

If you are dealing with a situation where expert evidence played a role in an employment or civil dispute, speaking with a lawyer early gives you the best chance of addressing the issue through the right channel. Our team at UL Lawyers also serves clients in the Hamilton and Burlington areas — you can learn more about our Burlington employment law services or our Hamilton employment law team if you are located in those communities.

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This article is automated commentary on a public court decision and is for general information only — not legal advice. Decisions rely on facts unique to each case. If you are affected by a similar issue, contact a lawyer for advice specific to your situation.