The discovery has sent the planning process into chaos with hundreds of other documents having to be checked to ensure their authenticity.
Planners were alerted by local man Brian Roche who received a receipt from the planning department thanking him for sending in a letter with his views on the development.
"I've never sent any letter in and never been to any meetings," he said. "My name was spelt wrong and the signature is nothing like mine," he added.
Councillors fear that the discovery could be the tip of the iceberg, and have called for an urgent investigation into the 600-plus letters both in support of and opposed to the plans.
Tesco's proposals, which would see it build a superstore almost twice the size of its current site, have been opposed by many locals.
Tesco had been distributing pro-forma letters in support of the plans to shoppers before the fake signature was discovered, and some of the unsigned letters may have been left unsupervised.
A spokesman for Tesco said the company was appalled that the system had been abused.