The UK’s competition watchdog has accused the US guitar maker Fender of fixing prices illegally by restricting online discounts on its guitars.

From 2013 to 2018, Fender Musical Instruments Europe impeded competitive online pricing by imposing a minimum sale price on vendors, the Competition and Markets Authority (CMA) said in a provisional ruling. The practice, known as resale price maintenance, is illegal and makes it difficult for customers to find good deals.

Fender is the world’s biggest maker of guitars and its famous models include the Stratocaster, Telecaster and Precision bass. The Arizona-based company’s models are played by stars such as Eric Clapton, U2’s The Edge and Taylor Swift, and its basic Squier range is popular with beginners.

The CMA has been investigating Fender since April 2018 and has already fined the company £25,000 for concealing notebooks at its European headquarters in East Grinstead. The watchdog fined Casio £3.7m in August for restricting online discounts for its digital pianos and keyboards.

Ann Pope, the CMA’s senior antitrust director, said: “Shopping online can make it much easier to compare prices and hunt down bargains – this can be especially important for potentially big purchases like a guitar.

“We take allegations of resale price maintenance very seriously because it removes one of the benefits of the internet of making it easier to quickly find a better price by shopping around.”

The CMA said guitars make up a big part of the UK market for musical instruments, which is worth £400m in sales. Online sales of instruments have grown to about 40% of the market, making it important that internet vendors can offer the price they want, according to the CMA.

It said it had not made a final decision that Fender broke the law and would listen to the company’s version of events before coming to a verdict by the end of the year.