Ticking clock: the rules on cause of action and limitation periods

In law, timing is everything. If the wrong deadline passes, a cause of action in a personal injury claim that is perfectly valid on Monday can be dead on arrival by Tuesday. This is because provincial laws set strict rules on when a case must be brought to court, and missing those rules does not just weaken it, but rather, ends it.

What is a cause of action in personal injury law?

A cause of action is the legal foundation of a lawsuit. It is the set of facts that gives a person the legal right to sue another party and seek a remedy through the courts. Without a valid cause of action, a court has no authority to hear the case at all.

In personal injury law, most causes of action are built on negligence. To establish a negligence action, a plaintiff must prove four elements:

• duty of care: the defendant owed the plaintiff a legal obligation to act reasonably
• breach: the defendant failed to meet that standard
• causation: the breach directly caused the plaintiff’s injury
• damages: the plaintiff suffered real, provable losses as a result

All four must be present. For example, a breach without causation, or causation without damages, is not enough.

Learn more about the elements of tort of negligence with this video:

Check out our article on “Personal injuries proceedings act: Limits, notices, and traps” to learn about the factors that lawyers and litigants should not miss in these cases.

The civil causes of action

Beyond negligence, there’s a wide range of other civil causes of action that are relevant to personal injury practice. In Ontario’s laws, for instance, this would include:

• premises liability under the Occupiers’ Liability Act
• medical malpractice, which is a specialized form of negligence
• strict liability under the Dog Owners’ Liability Act
• wrongful death claims governed in part by the Trustee Act

Each carries its own elements, but the underlying logic is the same: a plaintiff must show:

• that the law recognises their claim,
• that the facts fit the legal theory, and
• that real harm resulted from the defendant’s conduct

One point that trips up claimants is that a cause of action is not the same as a complaint or a Statement of Claim. While the cause of action is the legal theory, the Statement of Claim is the court document that describes it.

In addition, one Statement of Claim can contain multiple causes of action arising from the same accident. For example, both negligence and a breach of statutory duty may occur at the same incident that injured the plaintiff.

When does the two-year limitation period start?

Provincial laws govern the limitation periods for each cause of action in a personal injury claim. Under s. 4 of Ontario’s Limitations Act, 2002, a lawsuit must be commenced within two (2) years of the date the claim was discovered.

Missing the window under the limitation period allows the defendant to move to have the claim dismissed, regardless of how strong it is on the merits. As such, the word “discovered” is doing a lot of work here.

“Discovered” is not always the date of the accident

In straightforward cases, the clock starts on the date of the incident. An example is that if a driver runs a red light and injures a cyclist on a specific date, everyone generally knows that the claim arose on that date.

However, most laws on limitation periods are clear that the period runs from discovery, not necessarily from the accident itself. Under s. 5 of Ontario’s Limitations Act, it defines discovery as the earlier of two dates:

• the day the plaintiff knew that an injury occurred, that it was caused by someone’s act or omission, who that person was, and that a legal proceeding would be appropriate; or
• the day a reasonable person in the plaintiff’s situation ought to have known all of those things

This distinction matters enormously for latent injury claims, where the harm is not immediately apparent.

The ultimate limitation period: a hard backstop

Beyond the basic two-year period, s. 15 of Ontario’s Limitations Act sets an ultimate limitation period of 15 years from the date of the act or omission. It means that no proceeding can be commenced after that point, regardless of when the claim was discovered.

The one key exception is that the 15-year clock does not run if the claimant either:

• is a minor, or
• lacks legal capacity

More about these limitation periods with this video:

Check out Canadian Lawyer’s Special Report on Canada’s Best Personal Injury Law Firms for a list of the country’s leading personal injury boutiques and firms.

Notice periods can further shorten the window

For some claims in Ontario, shorter notice requirements apply well before the two-year period expires:

• municipal claims: if the injury was caused by a failure to maintain a road or sidewalk, written notice must be given to the municipality within 10 days of the incident under the Municipal Act, 2001
• snow and ice slip and falls: under the Occupiers’ Liability Act, written notice to the occupier is required within 60 days
• accident benefits: claims under Ontario’s Statutory Accident Benefits Schedule (SABS) operate on an entirely separate timeline from the tort lawsuit

Missing a notice period can be fatal to a claim even when the lawsuit is filed within the two-year window.

What is the discoverability rule and why does it matter?

The discoverability rule is the legal principle that delays the start of the limitation period until a plaintiff has, or reasonably should have, enough information to know they have a claim. It exists because not every injury is obvious the day it happens.

In Grant Thornton LLP v. New Brunswick, 2021 SCC 31, the Supreme Court of Canada settled what had been a long-running debate about how much a plaintiff needs to know before the clock starts ticking.

Justice Morris Moldaver, writing for a unanimous court, established what is now the governing standard. It was held that a claim is discovered when the plaintiff has actual or constructive knowledge of the material facts upon which a plausible inference of liability on the defendant’s part can be drawn.

How the discoverability rule works

That standard sits in a deliberate middle ground:

• more than a vague suspicion or speculation
• less than certainty or perfect knowledge of all the legal elements

Critically, a plaintiff does not need to know that the defendant owed a duty of care or exactly how the standard of care was breached. That level of detail typically emerges through the discovery process or expert reports, which only happens after a claim is already filed. What the plaintiff needs is enough facts to draw a reasonable inference that the defendant’s conduct may have caused the loss.

Medical malpractice: a common battleground

Among the other causes of action, medical malpractice claims are particularly prone to discoverability disputes.

The Ontario Court of Appeal addressed this directly in Brown v. Wahl, 2015 ONCA 778. In this case, the plaintiff had dental implants performed by two dentists. Following the procedures, she suffered pain, broken dentures, and infections.

A new dentist later explained the likely cause. She then commissioned an expert report, which concluded the original dentists had been negligent, and commenced proceedings based on that report.

However, the court dismissed the claim as time-barred. It held that the plaintiff had enough facts to draw the inference of negligence when she first consulted the new dentist, and not when the expert report arrived.

In other words, a formal expert opinion is not required to discover a medical malpractice claim.

Once discovered, the clock does not stop

As confirmed by the Ontario Court of Appeal in Har Jo Management Services Canada Ltd. v. York (Regional Municipality), 2018 ONCA 469, once a claim is discovered, there is no ongoing duty on the plaintiff to continue investigating it.

It means that the limitation period is simply a window within which the plaintiff must file a case in court. A defendant cannot argue the claim is out of time solely because it was filed close to the deadline.

Do minors and vulnerable plaintiffs get more time in limitation periods?

Yes; but the protection is not automatic, and there are important procedural steps that govern how and when it applies.

How the limitation periods protect minors

Provincial laws have their own provisions protecting minors from the strict requirements of limitation periods.

Under s. 6 of Ontario’s Limitations Act, 2002, the two-year limitation period does not run while a claimant is a minor and is not represented by a litigation guardian. A minor in Ontario is anyone under the age of 18.

In practice, the clock does not start until one of two things happens:

• the minor turns 18, at which point they have two years to commence a claim; or
• a litigation guardian is formally appointed to represent the minor in the proceedings

That second trigger is where things get procedurally complex, as the Ontario Superior Court of Justice illustrated in Siddiqui v. Saint Francis Xavier High School, 2019 ONSC 30.

Here, a father sent a notice letter stating he was acting as his daughter’s litigation guardian after she was injured at school. The defendant later argued this letter triggered the limitation period.

However, Justice Robert Beaudoin rejected the defendant’s argument, and held that simply claiming to be a litigation guardian in a letter is not enough. He said that the process set out in Rule 7.02(2) of Ontario’s Rules of Civil Procedure must be formally completed, including a sworn affidavit confirming consent, legal authority, and the absence of any adverse interest.

A note for defendants

Section 9(2) of Ontario’s Limitations Act gives defendants a tool that is often underused. This is their right to bring a motion to have a litigation guardian appointed for a minor plaintiff.

This right under the law terminates the suspension of the limitation period and starts the two-year clock. Without it, a defendant may face an open-ended exposure period lasting years.

Incapable persons receive the same protection

The same framework applies to persons who are legally incapable of bringing a claim due to a physical, mental, or psychological condition. The limitation period does not run while the person lacks capacity and has no litigation guardian. Again, a defendant can bring a motion under s. 9(2) to trigger the clock.

Any settlement reached on behalf of a minor must also be approved by the court under Rule 7.08 of the Rules of Civil Procedure. In such a case, the court must review two things:

• whether the amount is in the minor’s best interests, and
• how the proceeds will be held or disbursed; typically, in trust until the minor turns 18

Cause of action: when litigants should not wait

A valid cause of action is only as good as the time left to bring it. Provincial limitation rules are unforgiving, and the discoverability doctrine is not a safety net. Whether the case involves a latent injury, a child injured in an accident, or a claim against a municipality, the clock is always running somewhere. The only sure protection is early legal advice, prompt action, and understanding which deadlines apply.

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