Recipient’s name: [Name of person or organisation]:
Subject: [Describe the reason of the warning in less than 10 words]
Dear [Write down name of recipient with appropriate title],
[The first paragraph must make the most impact and should write in the very beginning why is the recipient of the letter being warned and about what. If it is any specific incident then the incident can be described briefly or if this is repeated behaviour, then mention what activities are unacceptable].
[In the second paragraph, the consequences of not paying attention to the warning may be given. If it is a warning to an employee then his employment may be terminated, if it is a tenant, his lease may be terminated and so on].
[In the last paragraph, you can sign off by mentioning the warning again and hoping for a positive change in behaviour from the receiver.]