By Josh Ong

Published: 08:20 PM EST (05:20 PM PST)

In an effort to avoid another public relations debacle, Apple is investigating scratching and cracking of the iPhone 4's glass panel back caused by third-party cases, a new report claims.

Sources inside and outside of Apple told Ryan Block of gdgt that the iPhone engineering team is in "quiet lockdown" as it works to solve the issue of potentially damaging third-party cases. Slide-on cases in particular may cause damage by trapping particulate matter between the phone and the case, eventually scratching, cracking, or even fracturing the glass back of the iPhone 4, Block wrote in his report.

"The iPhone team has grown to be very concerned by this issue with slide-on cases, and has created a lab and large new test program specifically to investigate this further," Block said.

Apple is anxious to prevent any further damage to the iPhone brand, especially now that coverage of the iPhone 4 antenna problem has finally quieted down. After reports of the iPhone 4 losing bars and dropping calls when gripped were picked up by the media, Apple chief executive Steve Jobs held a press conference in July to announce that the company would give away free bumper cases for every iPhone 4 through Sept. 30. The free case program may have cost the iPhone maker as much as $175 million in revenue.

After the free case program ended, Apple resumed selling its bumper cases and a select few third-party cases. However, Block points to a lack of variety in third-party cases, including a complete absence of slide-on cases, offered at Apple Stores as evidence that Apple is blocking sales while it solves the scratching problem.

In the meantime, third-party case manufacturers with slide-on case designs are reportedly "dead in the water" as they wait to get their cases into the Apple Store.

Apple representatives declined to comment on the story when contacted by Block.

In June, Engadget reported that an early review unit of the iPhone 4 had collected "incredibly minor" scratches after a few days of heavy use.