According to Which?, sales staff give out inaccurate information about the possibility of price increases.

The report found that 82% of shop assistants maintained that the price was fixed even when asked if it would stay the same throughout the length of the contract.

All shop assistants, when prompted, claimed that the features will stay the same throughout the contract.

Which? recently launched the ‘Fixed Means Fixed’ campaign, calling on phone companies to ensure that the price, and all aspects of fixed deals, remain the same for the full length of mobile phone contracts.

It says that in the past year, four out of the five main phone operators have taken advantage of a hidden clause that allows them to increase prices on contracts that appear to be ‘fixed’, a practice potentially netting the industry an extra £90m in a year.

Richard Lloyd, executive director at Which?, said: “It’s totally unacceptable that people aren’t being told the full story about potential price rises when signing up to contracts in mobile phone shops.

“Shockingly, even when we asked directly about price increases, the vast majority of staff denied this could happen.

“There should be no nasty surprises after signing a mobile contract. People must be confident that fixed really does mean fixed.”

Recent Which? research found that 70% of people on fixed contracts did not know that mobile phone companies could increase prices during the length of their contract.