An industrial cleaning solution that ended up in tea that a woman drank at a Utah restaurant was unintentionally mixed into a bag of sugar a month earlier, authorities say.

Jan Harding, 67, is in a critical condition with burns to her oesophagus and mouth after drinking the tea that day at Dickey's Barbecue Pit in South Jordan.

Corporal Sam Winkler of South Jordan police said an employee reported the mixing of the product that contained the chemical lye into the sugar bag on July 5.

A worker took large quantities of the substance thought to be sugar and mixed it into the iced-tea dispenser.

Cpl Winkler said police are waiting on Mrs Harding's condition before moving forward with any charges.