Shaun Springer, managing director, said he knew of at least two online sites where Napier Scott jobs appeared without authority.
Springer said: 'Our reputation is at risk. We have no control over what's happening. I also feel emotionally like an author whose work has been plagiarised.'
He said the integrity of established firms could be damaged by the rapid increase in the number of online recruitment sites, as a minority of these were behaving unethically.
But some online recruiters are unrepentant - including findajobinafrica.com, one of those where a Napier Scott job appeared without authority.
James Da Costa, a consultant at findajobinafrica.com, said: 'No, our site doesn't ask permission from other sites. Why should we? We provide a valuable service to job seekers.' He said the Napier Scott job has been removed from his site. 'But if people want to take us to court, they can jolly well do it,' he added.
Yngve Traborg, of the online site ClickAJob, has a similar view. 'The point of the internet is to make it easier to find things. We need to create transparency or we're still stuck in the Stone Age,' he said.
ClickAJob (which is not one of the sites cited by Napier Scott) provides links to jobs on many sites without permission.
In January, the recruitment firm Stepstone won a court order in Germany to stop ClickAJob doing this. But the company is appealing that decision, and says it has no intention of ending the practice on its UK site.
'I expect more sites will start doing it soon,' Traborg said.
Jonathan Riley, a solicitor at the law firm Lawrence Graham, said the legal position was often unclear. Copyright law could, however, be invoked to stop a firm simply copying job details, as opposed to providing a link. 'But every case is different and enforcement is another matter. You have to track down whoever's doing it and you have the bother of a court case,' he said.
Springer of Napier Scott said that self-regulation in the industry was the best way forward. 'I don't want to waste time suing people, but if it happens again, I might do,' he said.
He said rogue job adverts often brought applications from under-qualified candidates, which were time-consuming to deal with. His firm's good name could be compromised if it were associated with the wrong companies, he added.
Clive South, a spokesman for the Association of Online Recruiters (AOLR) said the law should be clarified and that the AOLR was trying to set standards for the industry. 'Copying of job adverts is a serious problem,' he said. The AOLR is unregulated and has about 15 members out of hundreds of online sites.