The study shows that this method produces an average 5.7 per cent response rate, compared to DMIS figures of 6.8 per cent for direct mail.
Adverts referring to an enclosed leaflet are the most effective with a 6.5 per cent response rate, followed by ads directing people to a phone number (six per cent). However, overall only 23 per cent of customer correspondence carries ads.
Credit card firms use it most, with 38 per cent including advertising messages. Insurance and fixed line telecoms are the next biggest users at 35 and 33 per cent, while mobile telecoms use it least (6.3 per cent).