A member of the public complained to the ad watchdog after seeing a television spot on 23 October, which advertised savings of up to £200.
The creative by Nice Biscuits featured an LG 50-inch plasma TV being offered for £499. The accompanying on-screen text claimed it had only been offered at the higher price of £699 for a week between 13 and 19 October.
In the same ad the retailer advertised £120 savings on a washing machine with text stating it cost £229, down from the higher price of £349 that it was available at between 6 and 19 October.
The complainant objected to the Advertising Standards Authority (ASA) on the grounds that the items had not been offered at the higher prices for a sufficient period of time to avoid consumers being misled by the price reductions.
Comet defended itself by saying it had abided by the pricing practices guide laid out by the Department for Business, Innovation and Skills by making it clear the higher price had not been available for 28 days beforehand.
However, the ASA said Comet had not obeyed guidelines because some of the goods being advertised were not on sale at the higher price for long enough to let consumers become aware of the price, view the goods and carry out a purchase.
The ASA ruled the washing machine was on sale at the higher price for a sufficient period of time, but objected to other products in the ad only being available for seven days at the higher price.
Consequently, it has banned the ad because the higher prices were not genuine retail prices and could be viewed as an attempt to mislead customers.
Follow Matthew Chapman at @mattchapmanUK