Adverts for the travel group which owns Thomson and First Choice have been banned for using "misleading" claims about savings on summer holidays.
Two websites, firstchoice.co.uk and thomson.co.uk, advertised holidays in Mexico during August last year, claiming offers of up to "27% off".
A reader, who had never seen the holidays offered at the implied higher prices, complained that the ads were misleading.
The Tui travel group said the price for package holidays changed frequently based on demand, but they set their base prices at the level they expected them to sell at.
However, due to a lack of demand in relation to the holiday in question, they determined that it would be appropriate to lower their prices in order to promote sales.
Tui did not believe the ads were misleading, and said that showing a discount was important to illustrate the relative value of different holiday packages.
The Advertising Standards Authority said consumers were likely to understand from the savings claims in both ads that the implied full prices of the holiday were the prices at which the holiday was usually sold by First Choice and Thomson respectively.
It said: "We acknowledged Tui's comments that holiday pricing often changed depending on demand. However, we considered the evidence provided had not adequately demonstrated that the holidays were usually sold at £2,048 and £1,557 by First Choice and Thomson respectively, and therefore the savings claims did not represent genuine savings against those usual selling prices.
"We concluded that the savings claims were likely to mislead."
It ruled that the ads must not appear again in their current forms, adding: "We told Tui to ensure that they did not make similar savings claims in future advertising unless they held evidence to demonstrate that the higher prices were the usual prices at which the holidays were sold, and that the savings claims reflected a genuine saving."