Tesco is limiting shoppers to three lettuces per customer due to a shortage of imported produce.
Courgettes, cabbage and broccoli have also been affected, while sweet gem and romaine lettuce have been taken off sale completely in some stores.
A drought, floods and freezing weather have depleted stocks for export from southern Spain, and a harsh winter has also affected growers in Italy, Greece and Turkey.
The crisis could even last until April if the weather in the Mediterranean does not improve, and prices would have to rise as a result.
Concerned healthy eaters have been sharing pictures of bare supermarket shelves with the hashtags #lettucecrisis and #courgettecrisis, while complaining that prices have nearly tripled in recent weeks.
While pet owners, who often buy lettuce in bulk for animals such as guinea pigs and rabbits, face finding alternative fodder.
Yorkshire retailer Morrisons is thought to have imposed a two-lettuce rule for customers.