Almost a third of people experienced inadvertent roaming on a daily basis in the worst affected areas of south Armagh and south Down.

The charges are received when a phone call or text message is made, or the internet is accessed in Northern Ireland, using a southern network.

Ofcom Northern Ireland director Jonathan Rose said: "While the European Commission has lowered the maximum roaming rates that operators can charge, our research shows that the problem of inadvertent roaming continues to be costly for tens of thousands of consumers in Northern Ireland."

In south Armagh and south Down, 30% of consumers encountered the problem every day but the problem was less prevalent in east Derry and north Antrim.

The latest European roaming regulation places a responsibility on mobile operators to take reasonable steps to protect their customers from inadvertent roaming charges.

Ofcom said it would write to operators to find out what actions they plan to meet the new obligation.

Lower price caps, introduced on 1 July by the European Union, further reduced roaming charges for voice calls and text.

The rules also saw the introduction of a cap on data roaming charges for the first time.

These will reduce the unit price of using roaming services and potentially lower the cost of inadvertent roaming, however, the overall cost to consumers may not decrease as usage continues to increase.

Sinn Féin MLA Phil Flanagan said that the latest report highlights the need for the immediate abolition of mobile phone roaming charges across the island of Ireland.

He added that some months ago he brought a motion to the Assembly calling for this to happen.

The Fermanagh/South Tyrone politician said the £300 bill some customers were paying annually for inadvertent roaming was just the "tip of the iceberg."

"The actual figure on the total spend on roaming charges across Ireland remains to be seen and it is a figure I will be pushing the mobile phone operators to make public.

"Residents and those operating businesses along the border corridor are being heavily penalised and it is unacceptable that this situation has been allowed to continue for so long."

He added that the flow of such vast sums of money out of the economy and into phone companies cannot be allowed.

Mr Flanagan said that over the last five years, there have been significant reductions in roaming charges thanks to action by the European Parliament.

He explained that the usage patterns of the typical mobile phone users "has drastically changed, with many more people now having access to smart phones and using substantial amounts of data on their phone for internet browsing, downloading files and accessing emails."

"It is therefore very useful to have this latest information to fully appreciate the extent of the problem, which clearly highlights the need for the immediate abolition of roaming charges across Ireland."

Mr Flanagan expressed his gratitude to Ofcom for taking the matter seriously and "putting the issue of extortionate roaming charges back on the agenda."

SDLP Foyle MLA Mark H Durkan has also called for mobile phone providers stamp out roaming charges for people living in border areas.

He wants a single tariff for customers to be introduced.

"People in border areas including Derry are being crippled due to the excessive charges being place on them by mobile phone providers particularly due to the rise in Internet browsing.

He added: "People are finding it tough enough to survive in the present economic conditions without being scourged with this extra expenditure."