Sports Direct has been warned to clarify its advertising after the regulator banned "misleading" and "unsubstantiated" savings claims.
The retail giant's website advertised a Puma Arsenal home shirt for £42 in February, claiming the price represented a saving of £17.99.
But a reader said the product had never been sold on Sportsdirect.com at £59.99, and challenged whether the price and savings claim were misleading and could be substantiated.
Sports Direct said the £59.99 claim was based on the recommended retail price (RRP) of the shirt, and provided a list from manufacturer Puma which showed an RRP for the kit of £60.
It also submitted a schedule of prices for other clubs, arguing that the evidence demonstrated consistent prices of between £55 and £65 for these types of shirts.
The Advertising Standards Authority (ASA) said consumers were likely to understand that £59.99 was the price at which the kit was usually sold on Sportsdirect.com.
The ASA said: "Because the evidence provided by Sportsdirect.com related to the prices of the featured kit, and other similar products, offered on other websites, we did not consider that it was adequate in demonstrating that the kit advertised was usually sold at £59.99 on their own website and that the savings claim represented genuine savings against that usual selling price.
"We therefore concluded that the price claim '£59.99' and the savings claim were misleading."
It ruled that the ad must not appear again in its current form again, adding: "We told Sportsdirect.com to ensure that they did not make savings claims that were likely to be understood as being based on the prices at which the products were usually sold, unless they held adequate evidence to substantiate those claims."