UK cigarette sales fell by 11% during July, the first month of the smoking ban in England, compared with a year earlier, research suggests.
The smoking ban came into effect in England on 1 July
The findings from Nielsen include all sales outlets. Previous figures showed a drop of 7%, but only covered sales in shops, not pubs and clubs.
AC Nielsen said July was an exceptional month and the combined drop of 7.3% in July and August was more indicative.
July's weather was notably bad and last year the World Cup helped boost sales.
The drop seen in July was less dramatic in August, when sales were 2.8% lower than in August last year.
One reason was the better weather, said AC Nielsen, as smokers were more prepared to go outside to smoke.
AC Nielsen said July's sudden slump echoed the pattern seen in other countries that have introduced a similar ban.
The aim of the regulation is to reduce the number of deaths from second-hand smoke.
A spokesperson for the organisation said it was too early to predict future trends, but added that the next big test would be the impact of new rules raising the legal age for buying cigarettes from 16 to 18, effective this month.
This will have implications for outlets with vending machines as well as those selling cigarettes or cigars from behind the bar, said AC Nielsen.