The Advertising Standards Authority (ASA) has launched an investigation into an ad by bookmaker Paddy Power guaranteeing to refund losing bets if South African athlete Oscar Pistorius is acquitted of murder.
46 complaints were made to the ASA about the promotion, which appeared in newspapers and on social media.
A petition on Change.org has also been signed by more than 115,000 people.
A spokesperson for Paddy Power said: "Despite the ASA announcing their investigation, we stand by our decision to run this betting market and publicise it to punters."
More top news
An "erratic" driver sparked a security alert after ploughing into a police car near the US Congress and nearly mowing down several officers.
Thousands of people have linked hands on Westminster Bridge in a show of solidarity - one week after the terror attack in the capital.
PC Keith Palmer, 48, stabbed by Khalid Masood while on duty in the grounds of the Palace of Westminster last Wednesday.