Mobile phone users on both sides of the Border are being hit with bills for roaming charges they are unaware that they are incurring.

A joint report by ComReg and Ofcom on cross-Border telecoms issues says the "nature and length of the Border mean that significant problems and costs are caused to users through inadvertent roaming to networks across the Border".

The report of a joint working group established between the groups which regulate telecoms on either side of the Border is the first since it was established last April.

It deals primarily with cross-Border roaming and data transfers, although it also proposes a new numbering system for Northern Irish phone users calling numbers in the Republic .

Research carried out by Ofcom indicates that the 43 per cent of mobile customers in Northern Ireland surveyed claim to have received a signal from the Republic while north of the Border.

About a third of these, roughly 180,000 people, used this signal, with almost 90,000 saying they were unaware of roaming costs.

Roaming costs are the charges for using another company's telecoms network.

The report notes that some operators had all-island roaming arrangements in place and others had been persuaded to introduce them by the working group.

However, no mobile phone operator has all-island arrangements in place for pre-paid customers even though they account for the majority of users - two-thirds in the Republic and three-quarters in the UK.

It says that, in Northern Ireland, such customers face roaming charges that are up to six times higher than those levied on post-pay customers.

Communications Minister Mr Dempsey said mobile phone companies needed to recognise the seriousness with which both governments viewed the impact of cost on cross-Border telecoms.

Along with the Northern Minister for Enterprise, Trade and Investment, Mr Barry Gardiner, he expressed concern at the report's finding that companies are paying a relatively high cost to transfer data across the Border via leased lines. However, they acknowledged that efforts were being made to tackle this issue.

Mr John Friel, regional chairman for the Federation of Small Business, said: "The high cost of cross-Border calls is a financial headache for any small business owner involved in cross-Border trade."

"Most small business owners rely on their mobile phone as a means of doing business. All mobile operators should offer all-island tariffs, including the pay-as-you-go option as well to customers."

Eircom has been ordered to outline how it will facilitate rival operators who want to unbundle the local loop - the connection from the nearest exchange to individual telephones - by February 15th.

Two separate directions, issued by ComReg, effectively rule out Eircom's proposal to exclude rivals from the framing of processes to introduce local loop unbundling and its effort to impose its own timeframe on the operation.

Operators accuse Eircom of blocking tactics to limit competition.