The National Association for the Defence of Consumers (CLVC) says that 23% of agents are not revealing their charges for sales and rentals.
In survey it checked some 36,000 adverts in shop windows and more than 4,000 online and found agents were not showing these charges despite it being obligatory under consumer law since 1990.
It says that as charges are negotiable this could make a significant difference in the purchase price or rental price of a property.
The CLVC says that people should not be fooled by an agent that simply puts frais dâ€™agence inclus (agency fees included) on an advert as this does not fulfil the legal obligation to set out the fees that will be charged.
The research also found that a large number of agents are also failing to include the DPE energy consumption for a property in listing which has been compulsory since 2011.
It found that although 85% of shop window adverts included the DPE rating, only 43% of internet listings did so. But the number of agents complying has increased since the CLCV carried out research in 2011 when only 43% of agencies including the DPE ratings in their windows and 20% online.
A spokesman said that the DPE ratings are important as they give a prospective buyer or tenant an idea of how much the property will cost to run. The association wants tougher action taken on agencies. According to the law they should be fined â‚¬38 for every advertisement that does not include the DPE details but few actually are.
Meanwhile a survey has found that 60% of buyers are being forced to delay their purchase due to finance problems. The research by the PAP group, which specialises in property listings between individuals, found that in a third of cases buyers do not have enough income, in a third they had difficulty getting a loan and 33% do not have enough of a deposit.