Serve Legal carries out purchasing programmes on behalf of retail groups to ensure store staff follow age-check policies. Staff pass the test if Serve Legal's visitor is required to provide official ID.
In the first six months of this year the company carried out 5,389 attempted alcohol purchases in the convenience sector, with a pass rate of 75%. This compares with a pass rate of 72% in 2009 and 65% in 2008.
However, small stores slipped behind the supermarkets, which achieved a pass rate of 78% from 4,568 visits an improvement on last year's 71%.
For cigarette purchases, the pass rate in c-stores shot up from 55% in 2009 to 71% this year.
Serve Legal director Charlie Mowat said that while c-stores were once perceived as easier targets for underage shoppers, independent shops which had adopted best practice programmes such as Challenge 25 had proved as "vigilant and proficient" at stopping underage sales as most supermarkets.
Mowat said: "As many c-stores become more professional, young kids will shift their focus to those operators who don't demonstrate best practice."