STATE-owned health insurer VHI has been accused of breaching rules put in place to protect consumers.
The insurer is running adverts which offer "two months free for thousands who sign up to One Plan Choice before April 26".
But consumer groups said customers do not actually get two months free.
Instead they sign up for a year and pay in 12 monthly instalments. Rather than paying nothing for two months, the overall cost of the plan has been reduced by the equivalent of two months' repayments.
The VHI denied this was misleading for consumers and said any suggestion that it was being less than transparent was "entirely bogus".
The offer price per adult for One Plan Choice is €740, but many people are confused by the advert, according to medical insurance expert Dermot Goode of healthinsurancesavings.ie.
He said people assumed there was a further two months reduced off the €740 adult price.
And chief executive of the Consumers' Association Dermott Jewell said there was a need to make the offer clearer to consumers.
"This has the potential to become very confusing. If people are being offered two months free then they should not have to pay anything for two months of the year."
But the VHI vehemently denied it was not being upfront with consumers. "The accusation that VHI has mislead consumers is entirely bogus," a spokeswoman said.
"Nowhere on the ad does it say or even infer that consumers will get a two-month payment holiday with this offer. In fact, it quite explicitly states that 'two months are free. Payment is across 12 months and cover is valid for one year'."
The spokeswoman said the statements in the advert were clear, unambiguous and not misleading in any way.
The normal annual price for One Plan Choice is €880 for an adult, €306 for a student, and €228 for a child, she said.
However, the special offer price is ten twelfths of each premium. This comes to €740 per adult, €255 per student, and €190 per child.
"Therefore, customers who take out this cover during the promotional period will receive 12 months' cover for 10 months' premium."
VHI also said it was happy that the advert was in line with provisions of the Central Bank's consumer protection code. Although the VHI is not regulated by the Central Bank it voluntarily follows the provisions of the consumer code. The code requires that all information being provided to consumers must be clear and accurate.
The VHI spokeswoman admitted it has had to make changes to adverts in the past, but said this was the case for its rivals also.
The Advertising Standards Authority said it had not received any complaints about the advertising promotion.