Billboards with the word “sex” in giant red letters have been banned after the Advertising Standards Authority (ASA) received more than 500 complaints.

The adverts, which promoted treatment for men with sexual problems, were put up by the Advanced Medical Institute (AMI) based in Twickenham.

The billboards asked, “Want longer lasting sex?”, with “sex” emblazoned across the whole length of the yellow background.

Below it, in smaller red letters, the adverts read: “Nasal delivery technology. Call the doctors at Advanced Medical Institute,” followed by the firm’s phone number.

The boards were sited across London, including near schools and in areas with a high Jewish population.

ASA said it received 522 complaints that the posters were “offensive and inappropriate for general public display”.

Many people considered the giant red letters of the word “sex” on the bright yellow backdrop as “unsubtle and crass” and “impossible to avoid”.

Some were embarrassed when their children asked them about its meaning.

AMI, which had willingly taken down the billboards due to the complaints, said only 11 per cent of men with sexual problems sought help. It believed the best way to reach them was through a “bold”

campaign.

The company also pointed out that the word “sex” had been emblazoned in large letters on British billboards for the films 'Sex and the City' and ‘Sex Drive’.

The ‘Sex Drive’ posters even used an image of a speed dial suggesting a man’s erect penis, with the slogan “coming soon”.

AMI said its billboards contained “no swearing, innuendo, inappropriate or suggestive imagery or nudity”.

It argued that their placement near schools could not have caused offence, as sex education for children as young as five is a Government priority.

There was also nothing in the adverts to offend Jews, whose religion permits sex within marriage, AMI added.

However, ASA found the style the adverts and their direct reference to sexual intercourse was “too stark and prominent”, and therefore had caused “serious and widespread offence”.

It also found that the billboards breached the advertising code because it indirectly promoted prescription-only medicine.

ASA ordered that the billboards should not be displayed in the same form, and that the phrase “nasal delivery technology” should be removed.

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."