In a statement to the stock exchange ahead of a trading statement due on Friday, Tobin said it was "time to move on".
Having led the company for over a decade.... the board and I agree that with the business in excellent health, now is the time... to explore other important opportunities and to make way for a new CEO to take the company to the next stage of its development.
The company said Tobin will step down at the end of October, but it will begin the hunt for a new chief executive immediately.
Tobin is known for his unusual management techniques: his senior management team has endured kidnapping by former KGB agents, swimming with sharks and even spending the night in an ice hotel. But it looks like his rather punishing strategy worked: since the company listed in 2007, its market cap has grown from £436m to £1.6bn.
Tobin will receive a "golden goodbye" of more than £750,000, plus a £70,000 payment to compensate him for statutory claims arising from the termination of his employment, and £10,000 (plus VAT) of outplacement counselling.