Licensing lawyer Graeme Cushion, of Poppleston Allen solicitors, said police sources had revealed an off-trade failure rate of 14 per cent compared to 20 per cent in the on-trade.
The 10-week police-led Tackling Underage Sales of Alcohol campaign, which started on May 4, saw police basic command units across England and Wales targeting up to 20 "problem premises" each, test purchasing each store up to five times.
All BCUs were invited to take part and 166 are believed to have participated.
The Home Office is not expected to release official results for weeks, but several police forces have published the results of their campaigns.
North Wales police reported no failures from 59 test purchases across on and off-trades.
Lead officer Supt Terry Kellaher said the campaign "shows working together with the licensing trade through schemes such as pub and off-trade watch really works.
"These strong partnership initiatives are reducing the opportunities for under-age sales and assisting in increasing reassurance within the community".
Westminster council carried out 21 stings and said two businesses - both off-licences - had failed, one on two occasions. Westminster - which will not identify the businesses for legal reasons - said both traders were issued with fixed penalty notices for each sale, and the shop which failed twice may have its licence reviewed.
Hertfordshire police's campaign saw only 22 failures out of 200 visits. Force licensing coordinator Inspector Deirdre Dent said: "It is extremely encouraging to see that the majority of the premises visited during the campaign did not sell to our test purchasers." Similar campaigns would continue to be run across the county, Dent added.
Humberside police saw a 9.5 per cent failure rate across on and off-trades, against 33 per cent earlier this year.
Responding to the TUSAC off-trade results, Thresher chief executive Roger Whiteside said: "I'm encouraged that the results have been a bit better but we still think there's more to do and we've got to keep going.
"I want to see our track record improving all the time - and it has - but we still got caught out in some places and we're not happy about it. We put an awful lot of effort into it which shows how hard it is."
Drinks retailers including Bargain Booze joint managing director Matthew Hughes and Rhythm & Booze managing director Martin Swaine have previouslycriticised TUSAC, telling OLN they believed problem areas rather than problem premises were being targeted.
The TUSAC off-trade failure rate compares favourably with previous blitzes which were not focused on problem premises. Last summer's campaign, which involved 5,447 shops and supermarkets, saw 21 per cent of premises fail.
Thresher has teamed up with
CitizenCard to offer customers half-price identity cards. Application packs are stamped with a store code, and a £5 reward will be paid to staff in that branch when the application is processed.