The introduction of plain tobacco packaging and minimum pack size legislation in the United Kingdom between May 2016 and May 2017 was accompanied by substantial changes in tobacco product diversity and pricing. This study investigates the extent to which these changes were associated with consumer switching between tobacco product types and price segments, and from tobacco to non-tobacco nicotine products. Longitudinal study (changing trends in product and price choices) and survey (reasons for consumer choices). Great Britain PARTICIPANTS: We used 11 695 British households from Kantar Worldpanel who purchased tobacco and non-tobacco nicotine products from March 2011 to December 2017. Product choice was defined using household's monthly purchases of tobacco (cigarettes, roll-your-own (RYO) tobacco, cigar/cigarillo and pipe tobacco) and non-tobacco nicotine products (e-cigarettes and nicotine replacement therapy), while price was defined using price paid by pack size and by price quartiles. Our switching analysis considered three switching alternatives: switching to a different tobacco product or to a combination of tobacco products, switching to any non-nicotine tobacco product, and switching out of our dataset. We explored changes in price quartile purchasing behaviour using binary variables for whether a household purchased or not from each price quartile monthly. Finally, self-declared consumer's choice was assessed through survey responses. The introduction of plain packaging and minimum pack sizes coincided with major switching to purchasing of pack sizes of 20 cigarettes or 30g or larger RYO tobacco (>99% of purchases in December 2017) and a migration towards lower-price cigarettes, as for cigarettes, around 80% of purchases were in the lowest price quartiles at the end of the study period compared to about 50% at the start of the study period. During the first 6months of implementation there was also a marked increase in the likelihood that cigarette smokers would switch to non-tobacco nicotine (OR 1.74, 95% CI: 1.18 to 2.57), predominantly e-cigarettes, compared with the period before May 2016. Survey results suggest that price was the main driver of changes in purchasing behaviour. Implementation of plain packaging and minimum pack size legislation in the UK appears to have been associated with tobacco users switching to lower price tobacco products and to e-cigarettes.