Apple has sent a letter to a patent holding firm that says its licenses extend to its app developers as well.

The letter was sent by Apple's senior vice president and general counsel, Bruce Sewell, to Lodsys, a company that holds several patents that it says entitle it to a license fee for the use of an in-app purchase feature that allows users to upgrade their apps. Lodsys told the app developers it wants a 0.575 percent cut of the app developers revenue.

Apple, in its letter, says, Apple is undisputedly licensed to these patents and the Apple App Makers are protected by that license. There is no basis for Lodsys' infringement allegations against Apple's App Makers. Apple intends to share this letter and the information set out herein with its App Makers and is fully prepared to defend Apple's license rights.

Lodsys CEO Mark Small did not respond to an emailed request for comment. In a blog that contains a FAQ about the issue, the company said it is seeking payment from app developers because they reap the benefits of the technology Lodsys says it has patents on.

Apple says that one reason Lodsys hasn't got a patent claim is the doctrine of first sale. The first sale doctrine says that once a copyrighted work (or patented piece of technology) is sold to a user, that user need not pay royalties, for the same reason that when one buys a book or CD one pays only once.

Lodsys originally started sending letters to app developers earlier this month. A few days ago the Electronic Frontier Foundation called on Apple to defend the app developers, as many don't have the resources to defend against a patent claim in court.

The full text of Apple's letter is here.