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Vodafone has announced a series of price hikes that mean customers' bills could be increased by more than 10 percent per month. The rises will not affect cost of the contracts themselves, but will increase the amount charged on usage that extends beyond inclusive monthly allowances.

Beginning on 28 June, the price rises will be introduced automatically, although Vodafone will calculate the estimated price rise per customer and alert those likely to see their monthly bill increase by at least 10 percent. Thanks to new Ofcom guidelines introduced earlier this year, Vodafone customers whose bills are set to increase by this amount or more are free to jump ship if they don't want to pay the extra. They just need to make sure they write to Vodafone telling them so by 28 June.

"We really hope you decide to stay with us, but as these changes have increased your monthly bill by more than 10 percent, you can end your agreement without charge," says the network in an announcement posted on its website.

The prices Vodafone customers will pay once they have exceeded their monthly allowances of texts, minutes and data are also being hiked up, which Vodafone explains by saying: "Like most businesses in the UK our costs are rising so we need to review our prices now and again." It's no secret that Vodafone has been struggling recently, and next year's earnings are widely expected to be hit hard by the significant investment the operator has poured into upgrading its network.

The effects of the price hikes can obviously be avoided by making sure you are on the right plan, and if you regularly exceed your monthly allowance, it might be time to rethink. Standard call charges are jumping from 40p to 45p, standard texts from 15p to 18p and UK data from £6 for 250MB to £6.50 for 250MB.

It's not all bad tidings, though. The good news is that Vodafone is also significantly reducing the cost of roaming charges in EU-regulated countries [Operators are legally obliged to abolish roaming charges in Europe by the end of 2015] . Making a call to another country will now cost 18.7p per minute, down from 24.5p per minute and sending a text message will cost 5.8p rather than 8.1p. Data charges are being slashed from 45.9p per MB to 19.8p per MB. These new rates will apply from 1 July.