The Gambling Commission’s chief executive on Thursday revealed it is “actively investigating” a number of alleged illegal bookmakers but said clamping down on black market operators such as the one revealed by the Racing Post this week was among the “hardest challenges” facing the regulator.

Andrew Rhodes, speaking to the Racing Post following his speech at the Betting and Gaming Council annual meeting in London on Thursday, called on people to contact the commission about similar cases.

It follows a Racing Post investigation into The Post Bookmakers, an unregulated firm which took bets through the messaging app WhatsApp and which claimed to have more than 1,000 customers, including people involved in racing.