Football fans lucky enough to be travelling to the football World Cup this summer are being warned that they could face huge mobile phone bills if they call, text, tweet or surf the web while in South Africa.

Many mobile phone companies count the country in their most expensive bracket when calculating the cost of mobile phone calls and texts, while accessing the web costs as much as £8 per megabyte with some operators. Football fans will run up that much data usage after accessing as few as 20 web pages, while any that download an "app" while abroad could run up far higher data charges.

Mobile phone operator 3 will this week slash its prices for customers going to the tournament, which starts on 11 June, while Vodafone has already announced it will extend its Passport cheap calls package to include the country.

But with more and more mobile phone users carrying smartphones that can access the internet, use social networking sites such as Twitter and Facebook, and send emails, the tournament could turn out to be very expensive for football fans who do not check first with their mobile operator.

"The rise of smartphones, like the iPhone, means that all it takes is a bit of browsing or a downloaded app and you are going to get absolutely stung if you don't know what you are paying for," warned Mike Wilson, broadband and mobiles manager at moneysupermarket.com. "The high cost of roaming is always an issue in the summer but the fact that the World Cup is being held in South Africa has upped the ante.

"The first thing that anyone considering travelling this summer should do is speak to their existing provider and find out what they are going to charge and whether there are any special deals that will reduce that amount. Don't forget to also shop around for better deals."

This week 3 will announce that it is dropping its data roaming prices by more than 50% during June and July. It will charge £1.25 per megabyte of data, which compares with as much as £8 per megabyte with Orange.

Owned by Hong Kong-based conglomerate Hutchison Whampoa, 3 is also dropping its call charges for South Africa – making calls from the country to anywhere else in the world will be charged at £1.40 per minute, while receiving calls will be 99p per minute and texts 25p.

While 3's data roaming charges are cheaper than its rivals, its voice prices are still relatively high. Both O2 and Vodafone charge less, while all four rival networks offer discounts to customers who sign up for special international calling promotions.

Vodafone's extension of its free Passport service to include South Africa for June and July means football fans who register before they travel can make calls from their existing bundles, after an initial 75p connection charge.

Vodafone has also dropped its data costs but only from 15 June 15, which is four days after kick-off in the first game. It currently charges £14.99 for 25MB of data a day in South Africa, but from 15 June it will charge £3 per megabyte for the first 5MB used by customers and £15 for every subsequent 5MB.