The phrase "a letter declaring" is correct and usable in written English.
You can use it when referring to a written document that announces or states something formally.
Example: "She received a letter declaring her acceptance into the university."
Alternatives: "a document stating" or "a note announcing".
Equustek sent the court a letter declaring their intention not to defend against Google's suit.
Finally, a week before the California primary, Brown released a letter declaring his support for Clinton.
Michael Palin, another patron, was one of 200 signatories from the arts and academia to a letter declaring support for the system.
Problematically, she joined the party after the worst of Moscow's purges and show trials, and even signed a letter declaring her faith in the guilt of the defendants.
Joseph Dunford, the chairman of the Joint Chiefs of Staff, promptly responded by sending out a letter declaring the old policy unchanged until Mattis could issue guidance.
Andrew Mason began his company's S-1 with a letter declaring that "Life is too short to be a boring company".
One day in 1988, the family received a government letter declaring that a railroad was going to be built through the neighborhood.
His top adviser, Arvind Kejriwal, published a public letter, declaring that the movement was at a crossroads and needed public guidance, a sentiment shared by many others who had been uncomfortable with the shift toward more overt politicking.
Mr. Holmes produced a draft letter, declaring Reuters's intent to terminate the contract immediately, despite a negotiated term that required six months' notice.
At one point, she wrote him an irritated letter declaring, "I am a young writer, and this is my life, and I'm not trying to be a model anyway".
The Council of Bishops of the United Methodist Church, the third-largest church in the United States, voted unanimously today to issue a pastoral letter declaring the bishops' "clear and unconditioned" opposition to any use of nuclear weapons.
