Since the launch of the iPhone 3GS, public perception of AT&T has progressively decreased, while opinions about competitor Verizon Wireless remained relatively unchanged, a new study shows.

In a daily survey of 5,000 people 18 and older, YouGov’s BrandIndex tracks companies based on factors of quality, value, satisfaction, recommendation, reputation and impression. When combining those categories, AT&T’s index score of 18.3 on June 16 had eroded to a 14.6 on Thursday — a change company senior vice president Ted Marzilli said is “statistically significant.”

He said the survey could indicate that some people may pause before they buy an iPhone because they don’t want to be locked into an AT&T contract. Though he believes the contract between AT&T and Apple has likely been beneficial to both parties for the last two years, the timing of AT&T’s public perception decline suggests the issue can be traced to the launch of the iPhone 3GS — or, more specifically, the network’s inability to meet the bandwidth needs of users with the device.

“It may be hindering iPhone adoption at this point,” Marzilli said, “and I think Apple is going to take a hard look at what they will do next.”

AT&T has always been perceived as an inferior brand to Verizon Wireless in the index rankings. However, it was in mid-June that the gap between the two companies began to widen. Though AT&T’s 14.6 score in overall feedback remains relatively positive, it still lags behind well behind Verizon in consumer perception. In the last three months, AT&T’s biggest hits came in the quality and recommendation rankings.