More than two in five mobile phone users have signed up to a two-year contract says Ofcom.
Research by the watchdog revealed that just a year ago, only five percent of handset owners opted for the 24-month contract, but this has surged in the past three months.
Ofcom said the popularity of Apple's iPhone, which is available without a handset surcharge on a two-year contract, was partly responsible for the increase.
Apple's iPhone has contributed to the surge in two-year contracts
The watchdog also said that networks offer the best value tariffs to those that are happy to be tied into a long contract.
Nick Hutton, from the government-funded Consumer Focus, told The Telegraph: "We are concerned that increasing contract lengths may limit consumer choice and could potentially lead to higher prices in the long-term. Longer contracts suit some customers and allow providers to offer discounts on handsets. However customers who find they need to leave their contract may face high termination charges. If consumers are tied into longer contracts and rarely switch it may also give less incentive for suppliers to offer the best price deal."