Vodafone received more customer complaints, as a proportion of its subscriber base, than any other mobile operator during Q4 2014.
Latest Ofcom figures reveal that the 3G and 4G mobile service provider witnessed 0.15 complaints per 1,000 customers between October and December.
This was up from 0.11 in Q3 2014 and 0.09 in Q4 2014 - a trend that may alarm Vodafone bosses.
In the final quarter of last year, the industry average was 0.09 complaints per 1,000 customers.
EE - which registered the worst score for fixed-line broadband in the Ofcom study - also put in a below-par mobile performance in Q4 2014
The firm received 0.12 complaints per 1,000 customers, with TalkTalk Mobile (0.11) only faring slightly better.
Virgin Mobile (0.07), O2 (0.04), Three (0.03) and Tesco Mobile (0.02) all received fewer customer complaints than the industry average.