Absolutely not (based on UK betting). You are not taxed on your online winnings, you can do whatever you like with it. While bookmakers are taxed heavily so the Government makes their money, it is too cumbersome for individuals to be taxed. For every win that would have to be taxed, they would have to offer tax relief to every losing bet. That’s not going to happen, so your winnings are all yours whether you are a casual or professional gambler.