The National Association of Head Teachers could face industrial action from its own staff who have claimed that management have been snooping on their emails.
One of the association's London regional officers was suspended in February for possible gross misconduct over the content of an email sent to colleagues.
Amicus, the union representing the staff, said that other NAHT employees have been threatened with disciplinary action following the "unauthorised and possibly unlawful" monitoring and interception of emails.
Barry Jones, Amicus regional officer, said: "The NAHT has no policy on email and internet use, and employees have been encouraged to use the email facility freely. It is clearly totally unacceptable for any employer, without warning and in secret, to monitor or intercept email traffic, still less to threaten staff with disciplinary action as a result."
He said an independent investigation had been refused by David Hart, NAHT, general secretary despite the presentation of compelling evidence.
This week Amicus lodged a formal complaint with the information commissioner and sought legal advice on the issue. The union says it will ballot NAHT employees on industrial action if the matter is not resolved satisfactorily.
"Morale is now at rock bottom among the majority of staff," said an employee who wished to remain anonymous. "This is a highly dysfunctional organisation."
Mr Hart said: "I entirely reject the allegations about unlawful monitoring and I will defend the NAHT's position if a complaint is made to the information commissioner."
The association's leadership is to be criticised in the conference's private session for accepting conference sponsorship from the fast food giant McDonald's.
Brendan Hassett, head of Dolphinholme primary, Lancaster, said the deal smacked of hypocrisy, coming at a time when there was so much concern over unhealthy school dinners.
Mr Hart said: "I don't think we are insensitive to any question about the NAHT's receipt of sponsorship money. But I think McDonald's are meeting concerns about healthy eating."