When frozen prawns started glowing in the dark at Halloween a startled Waitrose customer feared she was going mad.

Tina Thompson, 69, from Rutherwyke Close, Stoneleigh, had defrosted the supermarket's prawns for brunch so gave some to her cats and planned to feed the cats’ leftovers to the birds the next day.

She said: "I had completely forgotten about Halloween. We heard a group of children in the road and we turned the lights off."

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But the bid to avoid bother from trick-or-treaters had unexpected consequences when her husband noticed the prawns glowing on a saucer in the dark kitchen.

Mrs Thompson, a former Mayor of Wandsworth, said: "Just to make sure I had not gone completely mad I took this saucer nextdoor to show my neighbour.

"They were still glowing the next morning when I went down to Waitrose. I took them down in a box and said ‘if you go into a dark room you will see they are glowing’."

After the visit to Waitrose in Stone Place, Worcester Park, she received letters from the customer sales and support team, seen by the Epsom Guardian, in which they say that prawns are sometimes caught with plankton and the bacteria may be Pseudomonas fluorescens which is harmless.

But Mrs Thompson remains dissatisfied with their answers.

She said: "They have not told me what species of plankton or bacteria survives boiling and freezing. I don’t think it is an unreasonable request to know what I have eaten."

She even wondered if radioactive water from a leaking atomic power station in Japan could reach Indonesia where the prawns were farmed.

She said: "It’s probably highly unlikely but I feel I would like to be reassured. I would like to know the species so I could go to the Natural History Museum, contact the New Scientist or look it up online."

Mrs Thompson represented Southfields on Wandsworth Council for 16 years and was the Mayor of Wandsworth from 1997 to 1998.

She said: "I really don’t like being fobbed off. I’m a very determined lady. I wrote to Waitrose and said I was severely disappointed.

"If they are going to treat a loyal customer with such disregard, and what I consider contempt, I don’t want to shop there anymore. It’s a reasonable question, I don’t want them to sweep it under the carpet."

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A Waitrose spokesman said: "Bacteria is all around us and although we don’t see this occur very often but occasionally naturally-occurring, harmless bacteria (known as photobacterium) may be found on fish and shellfish.

"At certain temperatures and certain times of year - even after freezing or cooking - these may be seen to ‘glow’.

"We appreciate how this may be startling if someone sees this for the first time but would reassure any customer that this is harmless and is safe to eat.

"Nevertheless, if any customer is unhappy with any product bought from us we are happy to provide a refund and replacement.

"We take feedback on all our products very seriously and investigate this with our suppliers. In this instance, we believe this to be an isolated case."

"We have written to Mrs Thompson directly and would like to apologise for any concern caused."

After reports of glow-in-the-dark shrimp in the US state of Oregon in 2010, Kaety Hildenbrand, an Oregon State University sea grant extension specialist said the phenomena it is actually surprisingly common.

She said: "One person turned her lights off to watch a movie and her shrimp salad started to glow. Another man left his pink shrimp on the counter to thaw, and when he got up, it was glowing in the sink.

"People are calling to ask about safety and we’re assuring them there’s nothing wrong with their seafood."

Marine bacteria can cause luminescence when it grows on seafood products and salt added during process can exacerbate the trait.