"I think there are three things operators need to do: first of all, be very clear what they mean by 'unlimited' in advertisements; secondly, give advice to consumers so they know when they are actually reaching their limit; and thirdly, give people advice on the amount of data that's being downloaded."

Concerns have risen recently about people being hit with unexpectedly large bills after going online.

It is becoming more common for people to download bigger files, including songs, videos and emails with large attachments.

The ombudsman's office said some mobile phone packages offer "unlimited" downloads and consumers may believe their mobile phone bill will not go above a certain amount.

"Consumers may fail to realise that there is a small asterisk next to the word 'unlimited' which refers to small print stating that there is in fact a limit which, if exceeded, will incur further charges," it said.

The Advertising Standards Authority has looked into the matter and decided it is acceptable to use the term "unlimited", as long as the customer is made aware of the limits and that it does not affect the typical user, it added.

"As our technology develops, the 'typical user' is downloading and streaming more and more information. This is an emerging problem which we think could cause real consumer detriment - we want mobile phone companies to take action before it becomes more prevalent," the ombudsman's office said.

A spokeswoman for T-Mobile said its customers would never pay more than they expected for mobile data usage in the UK.

She said: "We take transparency of customer charges for data use in the UK very seriously. T-Mobile customers, whether on pay monthly, pay as you go or You Fix plans, will never pay more than they expect for their mobile data usage in the UK.

"For pay monthly customers, T-Mobile is the only UK operator to not charge customers run on rates for data use across all plans, meaning that even if a customer does reach the limit of their monthly data allowance, they can continue to browse internet and use email with no additional charge. That way they will always know how much they are paying and will never get an unexpected bill.

"Data usage for T-Mobile pay as you go customers is capped at a maximum of £1 per day - or if they are regular internet users on their smartphone they can opt for an internet booster (five days of internet = £2.50, 30 days = £5, six months = £20), so again customers will never have to pay more than they expect.

"You Fix, launched in August of this year, gives customers a fixed number of minutes and texts each month, plus one included flexible booster, all on a short 12 month plan.

"For any usage over and above the allowance, customers can simply top up as if on pay as you go. Whether the customer opts for the internet booster (in which case as per pay monthly, we will not charge any data run on rates), or opts for one of the other great boosters available (in which case they can top up via PAYG and use the internet for the £1/day max charge), they will never be paying more than they expect."

David Dyson, chief executive of Three UK, said: "Three's all-you-can-eat data plans offer customers genuinely unlimited internet use on their smartphones and no out-of-bundle data bill shock.

"Smartphone users typically sign up for two-year contracts and use increasing amounts of data as they learn just what their smartphone can do, making limited bundles an issue for many consumers.

"Both contract and pay-as-you-go customers tell us they prefer the peace of mind of all-you-can-eat data to fair-use policies or, worse still, out-of-bundle data bill shock.

"Consumers want to make the most out of their smartphones but many struggle to track their data use, our genuine all-you-can-eat plans take that problem away."

Three's all-you-can-eat data plans start from £18 per month for contract customers, £25 per month for sim-only The One Plan deals, and from £15 per month on pay-as-you-go.