High Street optician Optical Express has been admonished by Britain's data watchdog after thousands of customers complained that the company was sending them nuisance text messages.
The Information Commissioner's Office warned Optical Express to halt its actions, or else face further action.
An enforcement notice (PDF) was slapped on the Glasgow-based firm late last year, the ICO confirmed on Tuesday.
Optical Express was given 35 days to respond to the written warning, during which time it can launch an appeal against the complaint.
It sent out texts to customers that included details of a competition to win free laser eye surgery.
The ICO said more than 4,600 people had moaned about Optical Express to the Spam Reporting Service in the space of just seven months, claiming they had not granted the eye specialist permission to use their details for marketing purposes.
Sending such messages without first seeking consent from the customer is considered a violation of the Privacy and Electronic Communications Regulations (PDF).
The regulator's enforcement boss Andy Curry said:
We have issued this enforcement notice as a warning to the company that using people’s data without their consent is not acceptable. Any breach of the notice would be a criminal offence.