How To Write A Complaint Letter In Block Format . Attach copies of relevant supporting documents. Though you have a complaint regarding something and you are frustrated, but if you write the letter politely then it gives a good impression to the concerned person and chances of getting a resolution are also good. Pin on english from www.pinterest.ca It can either be a recommendation letter, complaint. A basic complaint letter format should have the following information: Dear ms brennan, i am writing this letter of complaint about a very serious matter and it is not only affecting me but my whole class.