A Sheffield poster company ran an illegal price-fixing cartel for more than four years, an investigation found.
GB Eye struck a deal with Birmingham rival Trod Ltd not to undercut each other on posters and frames sold on Amazon’s UK website.
And they stuck to it using repricing software, according to the Competition and Markets Authority.
A CMA spokesman said it was impossible to say how much the firms had benefited from the cartel, which ran from March 2011 to July 2015, but they typically increased prices by 20 per cent.
GB Eye will not be fined because it reported the cartel to the CMA and co-operated with the investigation. Trod has agreed to accept a fine of £163,371. The £15m turnover company called in administrators in March.
GB Eye is based on Russell Street in a prominent building beside the A61 Corporation Street in Sheffield city centre.
A spokesman said they had “no idea” they were breaking competition law and the cartel only applied to its online retail department which is a small part of its business.
He added: “We now recognise that the conduct was inappropriate. It is also important to appreciate that as soon as we became aware that the conduct was inappropriate we took immediate steps to report it to the CMA.
“We had insufficient knowledge of competition law and have taken steps to rectify that. We are now aware of the helpful compliance materials that the CMA has published and are making sure all relevant personnel are aware of that.
“The company has a clear commitment to compliance with competition law and all other laws that apply to our business.”
Amazon was not involved and has not been investigated.
Stephen Blake, senior director and head of the CMA’s Cartels and Criminal Group, said: “The parties used repricing software to implement an illegal agreement. Sellers on online platforms need to be aware that agreeing with each other to limit price competition in this way is illegal and can have serious consequences for the companies involved.”