The Office of Consumer Affairs has confirmed it received complaints about overcharging at Tesco supermarkets six months ago.
This appears to contradict a Tesco statement that overcharging uncovered in an RGDATA survey was due to the introduction of new information technology systems this year.
The Office of Consumer Affairs has denied it failed to act effectively on the complaints and says it may prosecute Tesco in relation to one of them.
An RGDATA survey last week found overcharging in each of the six Tesco-owned supermarkets visited by its researchers in Dublin and Wicklow.
According to Tesco, overpricing is "uncommon". Tesco also said that in normal circumstances the odds are 100,000 to one against an error being made in the price.
The Irish Times understands that the complaint made to the Office of Consumer Affairs by one consumer last October concerned a number of separate instances of overcharging.
According to RGDATA (the Retail Grocery, Dairy and Allied Trades' Association), it decided to survey prices at Tesco after receiving a complaint from a consumer about consistent overcharging over four months at Tesco-owned stores in Dundrum and Sandyford, Dublin.
The consumer had made a number of representations to the Office of Consumer Affairs but felt that "no substantive action" had been taken, according to a letter from RGDATA's director-general, Mr Michael Campbell, to the Director of Consumer Affairs, Ms Carmel Foley.
Another consumer told The Irish Times that overcharging had continued despite representations made by her to the Office of Consumer Affairs last year.
But a spokesman for the Office of Consumer Affairs said its investigations were ongoing. "There are a number of files in relation to pricing in respect of Tesco which are very active and have been since early October last year."
A spokesman said its inspectors had found "gross inefficiencies" in some Tesco supermarkets and it may take legal action against Tesco in relation to pricing. He said the Office of Consumer Affairs told Tesco yesterday it wanted the overcharging eliminated "within days".
In the RGDATA survey, the average overcharging was 3 per cent of the shopping bill. Mr Campbell says this amounts to £33.3 million of Tesco's Irish turnover and is costing each consumer an average of £4.50 a week.
In a statement Tesco Ireland said that "customers can be assured that overpricing incidences are uncommon and when brought to our attention are dealt with speedily and without question.
"Any incident of incorrect pricing is unacceptable and we take any such occurrences very seriously. However, in any large and complex grocery retailing system, errors do arise and are dealt with when detected."
The statement added: "This year, we have invested heavily in new IT/checkout systems which have involved people adapting to new technological and manual procedures. During this transition, it appears that a larger than usual number of price inaccuracies may have arisen and are being steadily eliminated.
"When incidences of overcharging are discovered or brought to our attention, it is our established policy to automatically reimburse customers on a `no quibble' basis and give the item free of charge to the customer."