An ENO spokeswoman said: 'Nicholls had a positive impact on the ENO and was well regarded.' Doran also praised Nicholl's work in a message to employees.
Both the ENO and Nicholls, however, declined to comment on the reasons for the resignation, which came at the end of Nicholl's probationary period.
The ENO said it is currently 'assessing the needs' of its communications department before deciding whether it will seek a replacement.
Before joining the London-based ENO, Nicholls spent 12 years at The Guardian, where she was head of press, PR and corporate affairs.
The ENO's public image has suffered from its delayed reopening in February this year.
It recently attempted to turn around its fortunes by decreasing ticket prices to enable the public to 'see a tenor for a tenner'.