The TV ad, which was broadcast on 23 November 2015, said customers could save £100 on a Toshiba laptop.
A viewer later complained to the Advertising Standards Authority that they had not been able to find an instance of the computer being sold at that price while the ad was being aired and challenged whether the ad was misleading.
When contacted by the ASA, Argos said it had 6,016 units of the laptop in stock, 4,798 of which were available in stores and that they had sold over half this number during the first week of the promotion, which began on 20 November.
By the end of the second week of the 12-day promotion, 537 laptops were left. Because they did not sell out, Argos said they believed they had made a reasonable estimate.
But, in its ruling today, the ASA said it understood that some customers who tried to buy the laptop could not do so because it was out of stock in some local areas, even if national stock remained.
The regulator said: "We were therefore concerned that Argos had not demonstrated that they had been capable of meeting actual demand, despite the left-over stock.
We understood that the complainant had been unable to find the laptop available in any of the stores that they had checked online, including stores that were a considerable distance from their locale."
The ASA ruled the ad was therefore misleading and must not appear again.