The Advertising Standards Agency (ASA) has admonished Groupon for misleading web surfers about online sales promotions.
In two separate rulings, the ASA upheld complaints brought against the privately-held e-commerce coupons website about various banner and Google-sponsored search ads.
The promotions included promises of steep discounts on train tickets, meals and flowers.
"We considered that consumers would understand those offers to be genuine and available to subscribers of the Groupon scheme," said the ASA.
"We noted that Groupon did not provide evidence that the offers were available or that they had been taken up by consumers. Because we had not seen any such evidence, we concluded that the ads were misleading."
Groupon blamed the creation of the banner ads on its German operation and said that it knew nothing of the misleading promotions until the ASA told the company about the complaints.
Despite the cockup, Groupon said that the responsibility of approving the sponsored search ads would remain in Germany.
It said the UK division of the company would train its German colleagues in the requirements of the CAP (Committee of Advertising Practice) Code "so that they could apply the relevant rules when creating future campaigns," said the ASA.
Groupon added that it recognised a need "to approve such banner ads to ensure that the content was relevant".
The ASA said that Groupon must cease advertising sales promotions if the firm was unable to demonstrate that they were genuine.
It told the company, which recently walked away from a $6bn takeover bid from Google, that the ads must not appear again in their current form.
This isn't the first time Groupon has copped a negative ruling from the ASA. In January this year the company promised not to republish adverts judged by the ASA to be misleading.
On that occasion, Groupon blamed its exponential growth that led to the rapid hiring of hundreds of new staff, and said the ads in question were written by new starters.