First a Hampshire family found a live juvenile frog in a bag of Waitrose mixed leaf salad. Then two dead frogs were found in bags of Tesco leafy spinach in separate incidents in London.
According to a statement from Tesco: "We are urgently investigating these incidents and working to establish which of our suppliers the products came from." A representative confirmed that the source was British. "Between late April and mid October, it is our policy to source 100 per cent of our spinach from the UK."
Meanwhile, Waitrose also said it was investigating the "isolated incident".
British Leafy Salads Association chairman Colin Bloomfield said: "It remains to be seen where in the supply chain they got in. But growers are being as environmentally friendly as possible and encourage wildlife. Frogs are not hazardous to public health in the way that, for example, birds can carry campylobacter. Growers take steps to prevent this but there is always the possibility of the odd one getting through."
He added: "In my experience, customers are quite sanguine about these things and would treat it as a one-off - it's not like E. coli where you can't tell whether there's a danger there or not."
Professor Trevor Beebee, president of the British Herpetological Society, said he was puzzled by the cluster of cases.
"I've never heard of it happening before," he added. "It's possible that frogs have been more active this year or there may be some behavioural thing underpinning it.
"But there's no evidence of increasing numbers - they've been fairly stable in recent years."