The Advertising Standards Agency (ASA) has banned William Hill from repeating two "misleading" and "unclear" promotions it ran in December.
The promotions were marketed as "risk free" and stated in their terms and conditions that customers would receive money back if they placed bets of £5 or £10 on "£5 Risk Free with Family Guy... Play Now" or "Christmas Cracker - Get £10 Risk Free on the Pig Wizard".
A customer complained to the ASA that the adverts were misleading as the £5 refund offered by the bookmaker was not available unless further money was also staked.
In a second case, a customer had been refused a refund after they supposedly had not put up a big enough stake and money they "won" during a game, which was later lost, was not counted as their own money.
"We noted that the T&Cs for the promotion appeared on the same page as the promotion itself meaning they were drawn to consumers’ attention. However, we considered that the wording of the terms was unclear and contradictory," the ASA said in its ruling on the first complaint.
"The ads must not appear again the form complained of. We told William Hill to ensure that terms and conditions were clearly worded and unambiguous, so that consumers would understand how the promotion worked and what amounts would be refunded. We also told it not to describe promotions as "risk free" if the mechanic of the relevant game meant that it was not possible in all cases to stake the required amount and receive a refund without a customer staking some additional money of their own," the ASA added.
In response to the ruling, a William Hill spokesperson said: "We note the ruling and will review the details, ensuring any lessons are learned with regard to future promotions."
William Hill's share price plunged over 13 per cent earlier this month after it warned of a £25m fall in online profits in 2016.
Bookmaking majors William Hill and Ladbrokes have both been hit hard by increased taxes over the past year, including the point-of-consumption tax, which targets online gambling profits, and greater regulation of fixed-odds betting terminals in branches.