Customers of major mobile networks including Three, Vodafone, EE and O2 have been warned by Martin Lewis that they could be overpaying – and now one network has responded with a promise.

Mobile firm GiffGaff has seen the 8 percent average mid-contract price hikes issued to customers in April and pledged not to do the same thing. It says it won’t increase prices midway through customers’ contracts.

Giffgaff currently offers unlimited minutes, data and texts for £35 a month – which equals…