The Advertising Standards Authority (ASA) said it would take down the ads itself after the Advanced Medical Institute (AMI), the firm behind the posters, indicated it would not willingly remove them.

The ASA said it had received over 400 complaints and was investigating whether the adverts, on 196 sites around the country, were offensive and inappropriate.

It added the ads could be illegal because they advertised a prescription-only medicine.

'We have instructed AMI to remove the ad primarily in the interest of public safety. If it refuses to co-operate, we will take action to have the ads removed,' the ASA said in a statement.

The watchdog, which acts to protect the public from misleading or offensive adverts, said it had the power to remove the billboard posters before it had completed its inquiries.

However the AMI said the call for it to withdraw what it described as an awareness campaign was 'bizarre' and that the ASA was reacting to 'hysteria and pressure from a relatively small group of people'.

'We're happy to cooperate with the ASA's investigation process, but it's important for all parties concerned that it is able to run its course,' said Dr Michael Spira, AMI Europe's Medical Director.

'We must not overreact. This isn't the first time the word sex has been used in an advertising campaign.'

The AMI, which provides treatment for men suffering from erectile dysfunction and premature ejaculation, said direct advertising was the best means of reaching its target audience.

'People in the community are very reluctant to discuss these taboo issues,' he said.

'We have found that the public awareness generated by our advertising enables men to feel less isolated, embarrassed and ashamed of their particular sexual dysfunction, and more motivated to pick up the phone and seek help.' -- REUTERS