The latest data indicates that in May the average weekly spend fell by 25% compared with the previous month.
The number of customer visits remained stable, at 2.6 visits per week, but the average spent was down from £29.09 to £21.68.
This decline was in turn partly caused by a drop in basket size, with fewer items bought per visit.
Lumina’s analysis is that with restrictions easing and restaurants, pubs and cafés re-opening, the share of meals eaten at home is falling.
Senior insight manager at Lumina Intelligence Katherine Prowse said: “As expected, the reopening of hospitality outlets and fewer travel restrictions has resulted in a change in shopper behaviour.
“There will continue to be a rebalancing in the share of occasions between in- and out-of-home, as shoppers return to their favourite pubs, bars and restaurants.
“Greater freedom of movement has also resulted in a shift to more transient, on-the-go shopper trends.
“Food-to-go has grown its share of missions, with these lower-ticket missions impacting spend, despite visit frequency remaining consistent.”