Halfords has been told to remove "misleading" pricing from its website after overstating savings on certain products.
The car and bike accessories retailer listed two car radios in July, claiming savings of £20 on each one.
But a complainant noted that the radios had only just been released, and challenged whether the savings claims could therefore be substantiated.
Halfords said the products went on sale at the original prices on June 17.
It had not intended to make any references to the higher prices in advertising because fewer than 28 days had passed before the sale price took effect, but said they had appeared in error.
The Advertising Standards Authority (ASA) noted that industry best practice recommended that comparative prices should have been available for at least 28 consecutive days and last offered no more than six months earlier.
It said the evidence supplied by Halfords was not sufficient to support the claimed savings for either model.
The ASA said: "We considered that Halfords needed to provide documentary evidence showing that the items had been sold at the higher prices listed in the ad for a sufficiently long period to ensure that they were genuine retail prices and that consumers were not misled by the savings claims.
It ruled that the claims must not appear again in their current form.
A spokesman for Halfords said: " This was a genuine mistake. We work hard at all times to observe the relevant guidelines and to provide our customers with accurate and helpful information."