Describing Ms Jenner as "a secondary reality television personality" the letter argued that, in contrast, Miss Minogue was an "internationally-renowned performing artist, humanitarian and breast cancer activist known worldwide simply as 'Kylie'."
Last week, the Patent Office rejected Ms Jenner's application.
Minogue has owned the trademark to kylie.com since 1996, the year before Jenner was born, according to The Mirror. She also owns copyrights to Kylie Minogue and other brand titles related to her albums and associated musicals.
Ms Jenner, who wants the name for her clothing and beauty empire, has already lodged an appeal.
It is a victory which will be a much needed boost for Minogue, who last week confirmed she had split up from her fiancé, British actor Joshua Sasse.
The I Should Be So Lucky singer was said to have called off their engagement because of suspicions he had become close to Spanish actress Marta Milans.