They are worried that the Financial Services Authority will interpret the regulations too strictly, forcing many ads to be amended or withdrawn.
The move is part of the FSA's plans to beef up the rules covering the retail financial services sector, which spent £1.4 billion on advertising and promotion last year.
"Our research shows that most consumers don't read the small print," Christine Farnish, the FSA's consumer director, said.
But agencies fear that the rules will lead to a period of uncertainty because they will not know what constitutes an adequate "health warning".
"If interpreted literally this will be a problem,
Andrew Porter, the chief executive of Masius, the D'Arcy group financial specialist, said.
The FSA says that it wants advertisers to keep to the spirit of the rules rather than tempt advertisers to find loopholes in them.
The new rules come six months after the IPA urged the Government to abolish "health warnings
in financial advertising.