The two stations, Cork 96FM and 103FM, originally offered separate services to Cork city and county, but their licensing terms were changed several years ago.

Each service is now broadcast throughout Cork city and the rest of the county.

Advertisers' complaints centre on the fact that, in the JNLR data, the bible for advertisers buying radio airtime, the 96FM and 103FM stations are still treated as a single entity, although separate figures are given for their combined city and county audiences.

Agencies claim they were not given an accurate picture of the Cork listening map, and may consequently have overspent by buying time on both stations. Advertising agencies were alerted to the anomaly in recent weeks by rival Cork station Red FM.

"We're a bit miffed and a little embarrassed about it, " said Liam McDonnell, chief operations office of Aegis, which owns Carat, Vizeum and Brindley Advertising and is Ireland's largest media buyer. "We've only ever had conjoined listenership data and we presumed there was one station with [local] optouts."

The change in the JNLR could have revenue implications for UTV, which owns both 96FM and 103FM. The current system would have "presented the two stations in the best light. . . UTV would have done very well out of it", according to one source.

The two Cork stations, which are a key element of UTV's radio business in the Republic, are reckoned to generate combined turnover of about 8m a year and are hugely profitable.

In 2003, when the stations had a turnover of 5.5m, they are thought to have generated operating profits of 1.8m.

At a meeting of the JNLR committee held last week to discuss the issue, it was decided that the survey, which is carried out by MRBI, is to be changed from April to show 96FM and 103FM as two separate stations.

UTV executives have asked to meet the committee to put their case for maintaining the status quo, but it is thought unlikely that the decision to separate the two stations will be reversed.