The phrase "a letter attesting" is correct and usable in written English.
It can be used when referring to a letter that serves as proof or confirmation of something, such as a fact or a statement.
Example: "Please provide a letter attesting to your employment history for the application process."
Alternatives: "a letter confirming" or "a letter verifying".
Eventually, the social worker wrote a letter attesting to Skylar's "gender dysphoria" but otherwise sound mental health.
He also asked both sides to sign a letter attesting to the intentions of the order, which Mr. Arpaio's lawyers said they would have to discuss before accepting.
Lindsay said she received a text from a friend Thursday asking if she would sign a letter attesting to Kavanaugh's character.
The men also paid $75,000 to Sidley Austin Brown & Wood, a major law firm, for a letter attesting to the propriety of the tax shelter.
PAGE C14 Kerry's Health Defended Senator John Kerry's campaign headquarters released a letter attesting to his "excellent health" as he prepared for outpatient shoulder surgery tomorrow.
He claimed the borrowing was inadvertent, and the school let him retake the course and put a letter attesting to his honesty in his file.
(One of the Beaumonts' sons, himself a doctor in England, wrote a public letter attesting that his mother had "received the very best care").
But folded into a neat square inside his wallet was a well-worn letter attesting that this was not so.
A strong quality letter attesting to your ability to succeed in the program will weigh more than a generic letter from a high-ranking professor.
Keep the syringes in their original package and carry a physician's letter attesting to your need for insulin to avoid customs problems.
Earlier, the Clinton campaign announced it would soon release additional information about the candidate's medical condition, going beyond a two-page letter attesting to her good health that her personal physician issued in July 2015.
