Bernstein said his decision to leave, after joining five years ago as the environmental pressure group's first director of communications and fundraising, was taken to move away from London.
He said: 'I haven't made any concrete decisions and am considering a number of options, including non-profit, independent consultancy, in-house and commercial agency roles.'
Since Bernstein joined FoE he has overseen a revamp of the organisation, developing its distinctive circular brand design and creating online campaigning.
He said: 'The aim has been to be more strategic, to have a clear message and target the right people at the right time. To achieve that it's also important that communications, fundraising and marketing functions come under one roof.'
Since the merger of the fundraising and communications departments FoE income has risen from £4m to £7m.
Bernstein, who heads a 35-strong team, added: 'We don't want to stop there and want to reach the £10m figure by 2005. That will be an important task for whoever replaces me.'
Before joining FoE he worked as a consultant for the RNIB and the NSPCC, where he was communications manager.