
Billboard adverts which used the word "Sex" in giant lettering attracted hundreds of complaints from the public.
The Advertising Standards Authority (ASA) upheld 522 complaints against the Advanced Medical Institute (AMI), which offers treatment to men suffering from impotence and other sexual problems.
The billboard posters used the slogan "Want longer lasting sex?", with the word sex emblazoned in large red letters.
The posters were placed across London and led to complaints that it was offensive and unsuitable for public locations. Others complained that AMI used billboards near schools and areas where they could be seen by children.
Twickenham-based AMI provides consultations and treatments for men who suffer impotence and premature ejaculation.
The ASA labelled the advertising campaign "unsubtle" and "crass".
The authority said it had caused embarrassment to parents whose children had questioned them after seeing the billboards.
A spokeswoman for the ASA said: "A number of complainants pointed out to us that the sheer size of the message made it impossible to avoid, which they found very uncomfortable.
"The ASA removed the advert pending investigation so we cannot tell how many more complaints we would have received had we allowed the advert to continue running. However, the nature of the advert shown on large billboards around London, led to over 500 complaints from people who felt the ads were crass and offensive. The ASA has upheld these complaints."
AMI Europe's Chief Medical Officer, Dr Michael Spira, said: "It is clear we were not out to offend anyone - and we know from representative surveys that the majority of Britons are not offended by our ads. We will appeal the decision as we believe all the evidence, including independent research, says the ASA has got this wrong."