Chairman Jonathan Choat said discussions had been ongoing at board level ‘for some months’ over the running of the agency.
‘We decided we probably only needed one person in charge, so Jim has decided to pursue other interests,’ said Choat.
‘I will effectively become chairman and MD in charge of the day-to-day running of the company. It is a practical decision and we’re sorry to see him go,’ he added.
Horsley was unavailable for comment, but is believed to be lining up a new role.