Katie Crabtree bought the pre-packed chicken, bacon and lettuce sandwich at the Stockport branch of the supermarket chain.
She had already bitten into the food and swallowed part of the creature before recognising its presence in the remainder of the sandwich.
A Tesco spokesman told FoodManufacture.co.uk: “We take any case of this kind very seriously.”
However, Crabtree has accused the supermarket chain of trying to ignore her.
'Nobody seemed to know'
When she failed to receive a response to her initial complaint, made both to the store and to head office, Crabtree reportedly had to contact the retailer several times. “Nobody seemed to know what was happening,” she claimed.
Finally, she said, Tesco sent her a letter of apology promising to investigate the matter. It also enclosed a £10 voucher.
Tesco said: “We have apologised to our customer and we are urgently investigating this matter with our supplier.”