Five top modelling agencies and a trade association have been fined a total of £1.5m for colluding to fix charges.
The Competition and Markets Authority (CMA) said they “regularly and systematically” exchanged information and discussed prices.
The biggest fine was given to the Storm agency, which has used models such as Kate Moss and Cara Delevingne.
Three of the agencies – Storm, Models One and Premier – said they would appeal against the fines.
The CMA said the five agencies got together regularly to agree on a minimum level of prices – which is against the law.
It also accused the Association of Model Agents (AMA), the trade association, of sending round emails to its members telling them to keep prices high.
However, the CMA stressed that big name “top models” were not involved in the price-fixing.
The CMA investigation looked at a wide range of modelling assignments, from small magazine shoots to big advertising campaigns.
It said that the customers affected included well-known High Street chains and online fashion retailers.
“Strong competition benefits consumers, the economy and society,” said the CMA’s John Wotton.
“In this case the agencies and the AMA colluded with each other over their approach to pricing and, in some instances, the agencies agreed to fix minimum prices.”
Storm Model Management was fined £491,000; Models One was fined £394,000, and FM Models has been told to pay £251,000, although it has since gone bust.
Smaller fines were given to Viva Model Management and Premier Model Management.
The AMA was fined £2,500.
John Horner, the managing director of Models One, said they had decided to appeal against the fines without hesitation.
“It is clear that even after a 20-month investigation the CMA has failed to understand our complex industry.
“The CMA is penalising modelling agencies for seeking to maintain professional standards within the industry, whilst also protecting the interests of young and vulnerable people.”
He said the level of the fine was out of line with the CMA’s own guidance.
The agencies said that any communication between them was more about usage and publication rights, rather than prices.
Yasmin Le Bon was among the models who spoke out in defence of the agencies.
“Had it not been for the knowledge, experience and professionalism of the agency I would not have sustained a career in the industry,” she said.
Hannah Cassidy, another well-known model, said she found the guidance offered to her by Storm was essential.
“Agents promote, but also protect, models from unfair contract and exclusivity terms which could harm their careers. They also make sure I get paid on time,” she said.