In Canada, under s. 48(1) of the Patent Act, a patentee can disclaim portions of an issued patent if "by mistake, accident or inadvertence, and without any willful intent to defraud or mislead the public," the patentee has "made a specification too broad, claiming more than that of which the patentee… was the inventor." Disclaimers can be filed to correct errors in patents and can be used to enhance the validity of existing claims by narrowing the claims based on newly discovered prior art. However, patentees should proceed with caution, as disclaimers filed in the wrong manner, or for improper purposes, can be fatal admissions against self-interest.
Recently, a Canadian court — Hershkovitz et al. v. Tyco Safety Products Canada Ltd. 2009 FC 256 — has held a disclaimer can be refused if not filed in the prescribed form and manner. The court noted that the patentee should follow the prescribed wording and must not use the disclaimer to redefine or broaden the invention claimed (the re-examination process is available for more substantive changes).
The Court also held (para. 78), despite the absence of a statutory deadline for filing a disclaimer, that a patentee has a duty to "act in good faith" to file a disclaimer "promptly and diligently when he or she becomes aware of a mistake, accident or inadvertence." A patentee should file a disclaimer as soon as the deficiency in the patent as identified and should not wait to file a disclaimer as a litigation tactic. Further, the onus of showing that there was "mistake, accident or inadvertence" falls on the patentee attempting to file the disclaimer. The patentee must be able to show (para. 79) that the disclaimer was filed "in good faith and not for an improper purpose."
If a disclaimer is refused, the patent at issue returns to the claims as they existed prior to filing the disclaimer, but the admissions made in the disclaimer regarding errors in the scope of the claims (i.e. that the issued claims were too broad in scope) will be worn by the patent (para. 94) "like a scarlet letter." Such admissions against the patentee's interest can be considered by the Court and can serve as the sole basis for finding of invalidity. Perhaps s48(1) should include its own disclaimer: "Use at Own Risk."