It is your responsibility to notify any claim or circumstance likely to give rise to a claim in accordance with the terms of the policy. Failure to do so may affect the validity of the claim.
Each insurer has their own requirements concerning the notification of claims but there is normally a specific condition detailing who you should notify under what circumstances and with what documentation.
In general terms, as soon as you become aware of any matter which could or may lead to a claim under the policy, you should notify the insurer with full particulars, copy documents, notices, communications, etc.
You should not deal with, make any admission of liability or attempt to settle a claim until you have the written agreement of the insurer.