Update: A spokesperson for Three has told techradar: "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."
Three also confirmed that all customers who have received a letter are out of contract, and there's no deadline for when customers will be contacted if they are affected.
Original story: Three is doubling prices for a number of its most loyal customers in an attempt to shut down older contracts.
Those on legacy tariffs are being switched to the 'closest' existing deal, but in some situations that can be double what customers are paying now.
One customer told techradar that their £15 monthly deal (originally £25, but with an added £10 loyalty discount) had now been increased to £33 a month.
The new deal has all you can eat data, and unlimited minutes and texts – but it no longer includes unlimited tethering, which is now restricted to 12GB a month.
Making the books right
It means the customer is now paying £18 more a month, for fewer features. Not all customers who are affected have been notified, and Three has confirmed the process is ongoing.
If your deal is set to change, you'll receive a letter detailing your options going forward. You have 30 days to decide to go with the new deal or cancel your contract entirely.
And it seems to be affecting a lot of customers. A lot of people are angry at the situation, and have been taking to the Three Twitter hashtag #makeitright to air their grievances.