Headteachers are searching the internet to find evidence of teachers' bad behaviour outside school to use against those they want to sack or discipline, union officials claim.
Any evidence is being used to "add gravitas" to allegations facing teachers, according to NASUWT executive members.
Teachers found posting photos of "inappropriate" behaviour on holidays or nights out, or having relationships with colleagues, are facing disciplinary action or warnings in school for breaking the code of conduct, the officials said.
They claim the General Teaching Council's (GTC) code, which was introduced in 2009, is leaving teachers' open to "intense scrutiny" in their private lives.
The code states that teachers have a duty to maintain "reasonable standards" in their own behaviour that enable them to "maintain an effective learning environment and also to uphold public trust and confidence in the profession."
But Ron Clooney, NASUWT executive member for the South of England, said the code is being held over teachers' heads "like a giant sword of Damocles" and is having a huge effect on their private lives.
He told the Times Educational Supplement: "Unscrupulous employers are hiding behind it when they want to get rid of a teacher. They are using it to try to add gravitas to allegations."
NASUWT executive member Mick Lyons said: "It has become common for heads to trawl through the internet and use what they find to discipline teachers.
"Other teachers are venting their spleen about their jobs on the internet and this is rebounding on them."
The union officials said cases had included teachers being disciplined for putting photos of themselves drunk, or wearing revealing clothing, on social networking sites.
A GTC spokeswoman said they had heard only two cases relating exclusively to teachers' non-criminal behaviour outside school, and both were before the code's introduction.
She said: "The code does not alter the threshold for disciplinary action and does not affect the way in which any cases referred - including those involving conduct outside school - are assessed.
"It is only if a teacher's private behaviour were to become public in a way that was damaging to the reputation of the profession, and if that behaviour were serious enough to warrant dismissal, that the GTC would become involved."
Ministers have announced plans to scrap the GTC and the decision has been welcomed by Chris Keates, NASUWT general secretary, who said most of its policies are unworkable.