At the time, Hit 40 UK was forced to deny rumours that the new producers would replace Fox. They had already created a pilot show hosted by T4 presenter June Sarpong. Fox pointed out that he had two years remaining on his contract with Hit 40 UK.
But Fox now says he is too busy with TV work, including a "huge prime-time format for Carlton", and will leave the chart show next spring.
Rajar figures revealed Hit 40 UK lost 307,000 listeners, 11% of its audience, over the six months to September.
Although publicly standing by the show, Woolworths, which signed a two-year £8m sponsorship in January, is believed to be unhappy with its performance.
No replacement for Fox has been announced, but Woolworths head of brand communications Mark Trinder said: "Whoever takes over will be another great name."