The ad by Jewels of London stated: "We need ladies and men urgently to accompany wealthy clients to events! Very high earning potential and the opportunity to meet new people, have fun and work hours to suit you! Limited vacancies..."
The ad did not mention that the would-be escorts would be charged an enrolment fee of between £200 and £500 when registering.
The Advertising Standards Authority launched an investigation after a complaint that the costs were not mentioned. It was also disputed that escorts were needed "urgently" or that there are "limited vacancies".
The ASA said: "We considered the existence of such a fee was information a consumer would need in order to make an informed decision about the vacancy, and that omitting the amount of the fee was likely to affect a consumer's decision on whether or not to enquire further.
"The amount of the fee was significant information, which we considered was not sufficient to be included only in the terms and conditions."
In a response, Jewels of London said the ad had been changed recently to include its website address, and that it asked all respondents to visit its website to read the terms and conditions, which stated clearly that it charged fees.
Jewels of London said it asked respondents to read the website before they would talk to them, so that everyone who wished to join it would be aware that fees were involved.
On the issue of "limited vacancies", the company said it was unable to prove their bookings because both their clients and escorts would not want them to divulge private information, given that they offered a very discreet service.
Even though the later version of the ad included Jewels of London’s website address, the ASA ruled the ad must not appear again in its current form and told the company it must make clear fees were payable on registering in future ads.
The ASA also warned Jewels of London not to imply it had current work opportunities available unless it held documentary evidence to show that was the case.