A leading watchdog is calling on mobile phone networks to do more to protect their customers from unexpectedly high phone bills.
According to research by Ofcom as many as 1.4 million people across the UK on a contract mobile deal have suffered a "bill shock" in the past six months.
A “bill shock” occurs when the amount you expect to pay for your monthly usage is far less than the amount actually displayed on your bill.
It can be caused by a range of issues like travelling abroad, using more data than you realised and unwittingly exceeding your bundled minutes and texts.
Ofcom's Bill Shock Review is good news for the customer. It aims to work with networks on ways to tackle the problem - and if that doesn't work it plans to look at regulations to force the industry to help.
While that won't reduce bills when charges have been rightfully incurred, the measures could include a range of automated reminders and warnings to stop people before their costs spirals.
Overseas travel outside of the EU is one of the biggest problems, especially with today's data-guzzling smartphones.
Holidaymakers returning from a break have often racked up bills running into hundreds and even thousands of pounds for downloading data, checking emails, viewing websites and using apps.
Those travelling within the EU are protected by rules forced on the networks that mean internet access must be cut off when it reaches a ceiling of 50 euros (£42).
Customers can then opt out of this or set a different limit beforehand, with alerts sent to phones when the bill is at 80% and 100% of the limit.
In addition, the watchdog is now proposing that UK mobile networks introduce voluntarily worldwide caps and alerts.
A spokesman said: "Ofcom believes mobile providers can do more to help customers control the amount they spend on their mobile phones.
"Ofcom has written to the mobile providers calling on them to do more to develop and promote ‘opt-in’ measures, such as tariffs that allow consumers to set their own financial caps or receive alerts about usage."
Another growing issue identified is lost or stolen phones. With street robberies often targeting the latest handsets - worth hundreds of pounds - owners can be left liable for a huge bill if that theft is not reported quickly enough and the phone cut off.
One suggestion by Ofcom is to limit the amount of liability for the phone's owner as well as simple ways to render handsets useless to criminals such as educating people how to lock their phones or wipe them if stolen.
Ernest Doku, telecoms expert at uSwitch.com, said: "At the moment, mobile phone users are often advised to switch off their data roaming when they travel abroad.
"With the fear of roaming lessened, networks should continue to see revenues from data roaming, as consumers are more confident about turning on their phones whilst on holiday."
Richard Lloyd, executive director of consumer champions Which?, added: "Ofcom's report confirms what consumers have been telling us for years - that bill shock is a big problem.
"We want clearer information for customers from the phone companies about what they charge for services in the UK and abroad, plus greater protection for consumers from hefty data charges."
Ofcom's findings come as Which? releases its report on the Best and Worst Mobile Phone Networks in the UK.
It questioned 8,000 users asking them to rate their network for satisfaction, with Asda Mobile again coming out top in the Pay As You Go category, beating big names such as Orange, T-Mobile, O2 and Vodafone.
In the Contract and Sim-only section, O2 was the customer favourite, only marginally ahead of Tesco Mobile, last year's No1.
Data-focused network Three came out on top of the high street stores, with Orange rated the weakest.
Below we offer some tips for preventing "bill shock" and getting the most of your mobile phone deal.
1) Be aware of your limits and charges. It sounds obvious and often it can be complicated to understand but just knowing how many minutes you have, how many texts and the level of your data allowance, is always the starting points. Most phones have a way to monitor your outgoing calls, texts and data usage so you can keep a check on it each month.
2) If you are going abroad, consider switching off data roaming on your smartphone. But remember this also applies to 3G-enabled tablets too. This will prevent the devices constantly downloading data in the background, racking up the cost.
3) Find out the calling, texting, picture messaging and data roaming costs of the country you are visiting before you travel. Data will be measured per megabyte. Outside of the EU, such as in America, these costs can rise massively. Speak to your network provider as they often have offers and bundles to help reduce these costs, but beware of signing up to anything that you'll need to keep, even when not abroad, as the total yearly cost could outweigh any savings.
4) Buying a local Pay As You Go Sim card can often work out cheaper. But remember, you'll then be on a totally different number and it may work out more expensive for those calling you from the UK. It might also not be compatible with your handset.
5) Always lock your mobile phone using the handset pin. Make sure to change it from the standard setting. Keep a note of your handset's IMEI serial number. You can find it by typing *#06# into the phone. Report any theft to the police immediately and also to your mobile phone insurance company if you have cover. Keep a note of your network's lost or stolen contact number somewhere other than on your phone, so you can notify them straight away to prevent being liable for any charges made.
6) If you're thinking of moving mobile phone providers, always shop around on the internet with the main providers. Online contract prices and deals tend to be cheaper or better value. You can check out any upgrade handsets in a store first before signing up on the web.
7) Be wary of so-called "free" gifts such as gaming consoles offered with a new mobile contract. The cost of it is likely to be added to your monthly fee - you don't get something for nothing.
8) Always make a note of when your contract is ending. Then always haggle when renewing your mobile phone deal, it's the only chance you'll get. Each network wants to keep you as a customer but they won't put the cheapest offer on a plate for you from the start. Find cheaper deals elsewhere with similar benefits and see if they will price-match.
9) Know your usage. Look back across your bills and check whether you've been using your full allowances or falling very short of bundled minutes and texts. There's no point paying for something you're not using.
10) If you do not agree with charges on your bill - whether they are ones you do not recognise or they are unexpectedly high - dispute them with your provider. You can also make a formal complaint or after eight weeks escalate it to the independent Alternative Dispute Resolution (ADR) scheme. http://consumers.ofcom.org.uk/tell-us/telecoms/adr/