Vodafone has announced changes to its pricing structure that could see customer bills rise by as much as 10%.
Beginning on June 28, Vodafone will raise the amount charged on usage beyond a customer's allowance, whether voice, data or texts, although contract costs themselves will not change.
Going forward, the cost of a standard UK call to mobile networks or landlines will rise from 40p a minute to 45p per minute. The cost of sending a standard SMS message will rise from 15p to 18p, with picture messages now costing 40p, up from 37p.
Regarding mobile data usage, customers will now be charged £6.50 for every 250MB they use, up from the current cost of £6 per 250MB.
The cost of calls to non-UK standard number will also increase, with customers now paying 20.4p a minute to call freephone numbers and non-geographic numbers, such as 0845 and 0870 numbers.
The company also announced it was cutting roaming costs in line with recent EU regulations, with the cost of a European call falling from 24.5p to 18.7p and text messages dropping from 8.1p to 5.8p.
Vodafone blamed the price increases on rising costs, saying, "Like most businesses in the UK our costs are rising so we need to review our prices now and again.
"You can avoid paying any of these charges by making sure you're on the right plan for you - with the right amount of minutes, texts and data - so you can enjoy a worry-free mobile experience."
"We promise we won't put up the monthly charge you pay for your plan when you stay within your allowance. This is our commitment to you for as long as your agreement lasts."
Vodafone will estimate how much bills will rise based on a three month profile of each customer, and will warn them if their bills are to rise above 10%.
But following new Ofcom rules earlier this year, Vodafone customers can leave their contracts free of charge, but those wishing to do so must write to Vodafone before prices are raised on 28 June.
In a statement to customers on its website, Vodafone said, "We really hope you decide to stay with us, but as these changes have increased your monthly bill by more than 10%, you can end your agreement without charge."
The company says that it is "committed to building Britain's strongest network", and is spending "more than £2.5m each day" on improving its UK network and services.