The move, which will see peak hours on its plans extended by between two and six hours, comes as O2's average revenue per user (ARPU) has levelled out due to increasing competition.

It will mean that customers of one O2 plan, Early Bird, will experience peak call charges throughout the night.

The difference in O2's call costs between peak and offpeak times is over 40c a minute, which has angered some customers.

One member of the popular discussion website, boards. ie, said the move showed that the company liked "squeezing their Irish customers to ensure their ARPU is one of the highest in the world".

"Reducing off peak hours is an effective price rise - let the consumer beware, " said another.

However, an O2 spokesman said the company denied the suggestion that it was trying to boost revenue.

"Most traffic takes place at times unaffected by the amended hours. On this basis, the majority of our customers will not be impacted by the changes, " he said.

He said the company was simply bringing its prepay peak and off-peak times in line with the rest of the industry and its own bill-pay products.

Last month, O2 revealed that annual ARPU from its Irish prepaid customers fell to Euro353 last year, compared to Euro360 the previous year. This compares with annual ARPUs of Euro105 from its German prepaid customers and Euro210 from its British ones.