The Portkey Office is a division of the Department of Magical Transportation in charge of managing the usage and maintenance of Portkeys. Consequently, it seems like a reasonable deduction that the Portkey Office is responsible for authorising the creation of Portkeys (and possibly may regulate their use once they have been created, as well).
Since Portkeys, according to Arthur Weasley, are often made from items that look like rubbish, and since there are restrictions on enchanting Muggle objects for fear of their falling back into Muggle hands at some future date, it seems reasonable that objects enchanted to function as Portkeys should be kept track of.
Behind the scenes
- It is possible that the Portkey Office is a relatively modern division. According to Wonderbook: Book of Spells, Orabella Nuttley, an employee of the Improper Use of Magic Office, was responsible for organising Portkeys for the race between Torquil McTavish and Silvio Astolfi in 1754, implying that the Portkey Office did not yet exist at this time.
- Harry Potter and the Order of the Phoenix (Mentioned only)