Thousands of Three mobile customers are set to see their payments double from £15 to £30 a month after the mobile network said it was scrapping a series of older tariffs.

The mobile giant, which has almost nine million UK customers, has written to all those who were on one of the terminated deals warning them that they will be migrated to an alternative price plan. The biggest losers are arguably those who signed up to the firm’s popular One Plan. Two years ago this was one of the best available deals offering 2,000 minutes, 5,000 texts and unlimited data, all for £15 a month.

The company says it is moving these and other customers on to “the closest plan to the one they are on”. In many cases this means customers will be moved on to a plan that usually costs £33 a month for unlimited minutes, texts and data, though those being migrated will pay £30 a month.

A Three spokesman says: “In March 2014 we introduced new price plans giving customers more options in the size of their data and voice bundles, as well as limits and alerts to prevent bill shock. We have a lot of tariffs that we no longer sell, and moving customers to one of the new plans will ensure they can enjoy the benefits of these plans.”

Affected customers can quit without penalty for a different provider, or move to another Three deal – and have 30 days from the letter’s date to act. Those that do nothing will be automatically moved on to the new one-month £30 rolling contract.

Many users may not need unlimited calls and data and could be fine on a cheaper tariff either at Three or with a rival provider. Those people calling Three’s customer services (0333 300 3333) and asking to leave are being offered improved rates to stay.

Other providers offering unlimited calls and data include Giffgaff, which has a £20-a-month sim-only offer, while Tesco Mobile has 1,000 minutes and 2GB of data for £12.50 a month, again sim-only but on an 18-month contract. Switching is easy and you can take your existing mobile number with you.