Guitar maker Fender has been fined £4.5m for illegally preventing online discounts on its products in the UK.
Fender Europe required sellers to set a minimum online price, breaking competition rules, the Competition and Markets Authority said.
This meant that customers could not shop around for the lowest price, the regulator said.
Fender said it cooperated with the investigation and has changed its practices.
The company, founded in California in 1946, is famous for its guitars, which have been the choice of musicians ranging from Eric Clapton to Jimi Hendrix and David Gilmour.
Jimi Hendrix was one influential player to have used Fender guitars
The Competition and Markets Authority (CMA) said its investigation concerned conduct between 2013 and 2018,
The fine is the largest penalty the regulator has ever handed down for such behaviour. It follows a £3.7m penalty for keyboard maker Casio Electronics for similar practices.
“It is absolutely essential that companies do not prevent people from being able to shop around to buy their products at the best possible price,” said Andrea Coscelli, the CMA chief executive.
“This is especially important for expensive and popular items like guitars, and so Fender’s actions could have had a big impact on customers.”
Guitars account for a “significant” part of the UK’s £440m in total instrument sales - 40% of which take place online, the regulator said.
Those statistics make it “even more important that musicians have access to competitive prices online,” the regulator said.
A Fender Europe spokesman said: “As a result of the investigation, we have taken additional steps to enhance our compliance procedures and our new EMEA executive team will continue to rigorously monitor the business’ adherence to antitrust and competition laws.”
Fender Europe, based in West Sussex, was fined £25,000 in March after an employee hid notebooks related to the CMA investigation.
That employee no longer works for the company.