The Information Commissioner (ICO) has fined mobile network EE £100,000 for sending text messages to customers without their consent.
The ICO says more than 2.5 million messages were sent in 2018.
The purpose of the texts was to encourage EE customers to use the firm's app and also to upgrade their handsets.
EE said it believed at the time that the messages were service messages rather than direct marketing.
However, the ICO ruled that if a message includes promotional material then electronic marketing rules apply, and the fine for not complying with this legislation can be up to £500,000.
"These were marketing messages which promoted the company's products and services," said Andy White, director of investigations at the ICO.
"The direct marketing guidance is clear: if a message that contains customer service information also includes promotional material to buy extra products for services, it is no longer a service message and electronic marketing rules apply."
EE said in a statement that it accepted the ICO's findings and aimed to improve its processes.
"We're committed to ensuring our customers are fully aware of their options throughout the life of their contract, and we apologise to the customers who received these messages," it said.