The Information Commissioner's Office has fined Fareham telco Onecom Limited £100,000 for sending spam texts.
The unwanted messages for mobile phone upgrades were sent between late 2015 and June 2016.
The ICO received 1,050 complaints about the spam campaign. Although 944 of the messages did not identify the company, the ICO is confident they all came from Onecom.
Steve Eckersley, ICO head of enforcement, said: "Spam texts are a real nuisance to millions of people across the country and this firm's failure to follow the rules drove over 1,000 people to complain."
The messages were sent to customers of Three, O2, EE and Vodafone, and promised free tablets or smartwatches for anyone upgrading their handset.
Onecom confirmed that it had sent 3,284,908 text messages between 1 October 2015 and 31 March 2016. Of these, 2,796,075 were received.
Onecom was unable to prove that it had subscribers' consent or could rely on "soft opt-ins" – where a company gets a number from some other sales activity.
If Onecom pays up by 12 June, the fine is reduced to £80,000.
In a statement, the business said: "Onecom operates in a highly regulated sector and is utterly committed to upholding the highest standards in all our communications with our clients and the public. Consequently we consider this incident to have been most serious and have undertaken an internal review which has resulted in our further tightening our systems and controls to ensure that there is no repetition. We are pleased to note that no subsequent complaints have been received and that the Information Commissioner has acknowledged our remedial measures."
You can report spam texts by forwarding the message to the GSMA's reporting service on 7726 – which spells SPAM. ®