Appeal from an order of the Supreme Court (Rose, J.), entered May 9, 1997 in Tioga County, which, upon reargument, inter alia, denied defendant’s cross motion for summary judgment.
On February 28, 1994 plaintiff James E. Owen (hereinafter plaintiff), as a pedestrian, was struck by a vehicle owned by Lois Williams and Albert Williams; approximately 10 to 15 minutes after the accident plaintiff noticed a sharp pain in his lower back. On March 1, 1994, plaintiff notified defendant, his insurance carrier, of the incident and also sought medical treatment for his back pain. Notably, during the period of time in question, plaintiff was a licensed insurance agent in New York and an authorized agent for several companies including defendant; he had written his own automobile policy with defendant. During the period between February 28, 1994 and February 1995, plaintiff noticed a gradual worsening of his back pain and eventually underwent back surgery in March 1995. In July 1995, plaintiff made a claim against the Williamses’ *1019automobile insurance carrier, State Farm Insurance Company, and was subsequently paid the policy limit of $25,000. In August 1995 plaintiffs counsel informed defendant of his intent to commence an underinsurance claim against defendant; within days defendant disclaimed coverage on the ground of late notice.
Plaintiff and his wife, derivatively, then commenced this declaratory judgment action and moved for summary judgment; in response, defendant cross-moved for summary judgment. Supreme Court ultimately denied both plaintiffs’ motion and defendant’s cross motion for summary judgment on the ground that plaintiffs deposition testimony raised an issue of fact as to the timeliness of the notice of underinsurance claim. Defendant appeals.
It is settled law that an insured must give his or her insurer notice of an underinsurance claim within the time specified in the insurance policy or within a reasonable time after the occurrence of the incident upon which the claim is based (see, Matter of Nationwide Mut. Ins. Co. [Oglesby], 219 AD2d 771) and such notice is a condition precedent to coverage (see, White v City of New York, 81 NY2d 955, 957). When the terms of the policy require that an insured give notice of a claim “as soon as practicable”, the insured must give notice “ ‘ “within a reasonable time under all the circumstances” ’ ” (Matan v Nationwide Mut. Ins. Co., 243 AD2d 978, quoting Matter of Nationwide Mut. Ins. Co. [Oglesby], supra, at 771, quoting Matter of Preferred Mut. Ins. Co. [Sullivan], 199 AD2d 719, 720). “The reasonableness of the notice must be determined on a case by case basis” (Matan v Nationwide Mut. Ins. Co., supra, at 978-979 [citation omitted]). Under certain circumstances, late notice of a claim may be excused; the insured, however, bears the burden of demonstrating the reasonableness of such excuse (see, White v City of New York, supra, at 957).
Here, plaintiff was injured as a result of a motor vehicle-pedestrian accident on February 28, 1994 and reported the incident to defendant on March 1, 1994. Plaintiffs contract of insurance with defendant provided that any claim for underinsurance coverage must be reported, in writing, “as soon as possible”. Although plaintiff’s condition was deteriorating and he was aware of the identity of the Williamses’ insurance carrier, he did not seek information about the policy limits until July 1995 and did not give notice of an underinsurance claim to defendant until August 16, 1995, nearly 18 months after the accident. In our view, plaintiffs delay in ascertaining the Williamses’ policy limits for upwards of 15 months after the *1020accident constitutes a lack of due diligence (see, Matter of Allstate Ins. Co. [Dewyea], 245 AD2d 667, 668). Moreover, he was aware of the Williamses’ policy limits for approximately six weeks before notice was given to defendant; we conclude that this delay was not “as soon as possible” and unreasonable as a matter of law (see, id., at 667-668; Matter of Nationwide Mut. Ins. Co. [De Rose], 241 AD2d 607, 608).
Mercure and Crew III, JJ., concur.