The Consumers' Association (CA) is campaigning against the plans which will be considered next week by the European Commission (EC).
The organisation has written to EC officials, and to the UK Government, urging them to put public interest before the demands of the pharmaceutical industry.
A CA report says that the NHS drugs bill could go sky-high if drug advertising and marketing appears in mainstream British media.
However, the drugs industry says the proposals are not designed to allow drug advertising, but to enable people to obtain information about products if they want it.
The EC proposals would relax the rules on promoting drugs for Aids, diabetes and asthma by allowing pharmaceutical companies to create interactive patient information websites and advertise their treatments for those conditions.
US lesson
The CA points to the impact of advertising in the US, where the drugs bill increased by 84% between 1993 and 1998.
It is also worried that there is very little good quality information available to patients about prescription drugs.
Pharmaceutical companies say that advertising will fill this gap. But CA research has found that advertising information is generally of poor quality.
In a recent CA survey, only 6% of respondents said they would trust drug companies to provide accurate information on the best treatment.
The research also found that advertising is targeted at a narrow band of conditions and the drugs available to treat them - treatments for allergies, baldness, ulcers, and cholesterol control for instance.
Awareness
The CA is concerned that advertising would not raise awareness of a wide range of health conditions and treatments. The most advertised drugs become the most popular - even if a competitor or generic drug performs better.
Clara Mackay, principal policy adviser at Consumers' Association, said: "Governments must put the consumer first - particularly on crucial areas such as public health.
"Current restrictions in advertising prescription drugs to the public should not be lifted.
"The US and New Zealand experiences show that the billions of pounds spent on marketing drugs has a monumental impact on the drugs bill.
"Patients need better information on drugs and treatments. However, it is not appropriate for the pharmaceutical industry to provide this."
Drug company response
Dr Trevor Jones, of the Association of the British Pharmaceutical Industry, said the proposal was not about advertising, but getting better information to patients who wanted it.
Nobody was seeking to allow US-style advertising.
He said: "There are very strict controls by law of what can be said about medicines.
"What this proposal is about is to say if a patient wants to know more information then pharmaceutical companies are entitled to give it.
"At the moment there is all sorts of junk on the internet, and all sorts of misleading information we are prohibited from giving quality information."