"SCARE tactics" have been deployed to prevent councillors from even expressing their opinions about private firms which are bidding to take over council services.
Members were warned they could face legal action if they criticise the two companies.
County Hall legal chief Richard Williams asked councillors to get any statements they make about BT and Computer Sciences Corporation (CSC) checked by staff after BT complained about things said during last week's full council meeting.
Independent councillor Graham Walker had highlighted local government projects run by BT in Suffolk and Liverpool which had not been as successful as others run by the firm.
He had mentioned them to balance a list of successful examples cited by Cabinet member Steve Double to show councillors that private partnership schemes of the type being considered by Cornwall Council had a good track record.
Mr Walker suggested it was easier to find "failing" projects than it was to find successful ones.
In an e-mail sent to all councillors last week Mr Williams said: "Expressing opinions which are unfair, inaccurate or both is disrespectful to the companies concerned and may lead to legal action being taken against the i ndividual member."
He then listed four "areas of concern" that had been made by BT including that such comments could "demotivate" staff and that "this language is not helpful to the mutual aim of the council and BT in seeking to stimulate jobs and investment for Cornwall".
The message sparked an e-mail debate by members, seen by the West Briton, with one councillor saying they "deplore the scare tactics being undertaken by Cornwall Council".
But Cabinet member Graeme Hicks said: "I do not accept that these are scare tactics; I believe they are offered to you in very good faith."