Milligan joined Camelot from Kellogg's in 1998 and was the company's chief marketer during its bid for a second seven-year term as the National Lottery licence holder. He was drafted in to replace Diane Thompson, who was promoted to chief executive.
It is thought Milligan does not have another job lined up. He said he is tired of commuting from his home in Manchester to the company's offices in Watford.
Camelot is said to be looking for a commercial director to sit on its board. The company is also recruiting a sales director and a marketing director, both of whom will report to the commercial director.
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