Valerie Tomlinson, 53, was given a final written warning by the Royal Cornwall Hospital Trust after a disciplinary hearing she attended last week.
The theatre sister carried out three parts of the operation on 19 December at Treliske hospital in Truro. The surgeon, Tahir Bhatti, carried out the most difficult elements. He supervised Mrs Tomlinson, who was acting as his first assistant, and was present at all times.
Brian Milstead, the trust's chief executive, said Mrs Tomlinson, of Roseannon, near St Wenn, had demonstrated an insufficient understanding of trust and professional practice policies and would have to undergo a period of professional development and clinical supervision. However, if she received a satisfactory assessment she could be back in the operating theatre in six months.
Mrs Tomlinson, a nurse for more than 30 years, was "very relieved at what I think is a fair decision. I am glad it is all over, it has been a bit of an ordeal." She said that what happened took place "as a result of mutual trust and understanding" between her and Mr Bhatti.
The inquiry established she had breached guidelines of the National Association of Theatre Nurses, and Mrs Tomlinson admitted she was wrong to carry out parts of the operation, for which she had not been formally trained. The safety of the male patient -who was told about what happened - was not compromised. He has not complained.
Howard Catton, regional officer for the Royal College of Nursing, said Mrs Tomlinson had an exemplary career with no disciplinary record. "This was not a have-a-go nurse beside the operating table having to be restrained," he said.
How she came to carry out part of the operation was not discussed as the report has to be considered by Mr Bhatti's employers, South and West Regional Health Authority. He is on paid leave.
Libby Campbell, National Association of Theatre Nurses chairman, said: "It was an unfortunate thing to happen but if anything good has come out of it, it isthe opportunity for raising the whole issue of developing theatre nurses' role."
She said experienced theatre nurses could be just as able as medical students. "But any change in role must include proper education, assessment and monitoring and the employer and nurse must agree a job description."
The Royal College of Nursing added that it was already quite routine for nurses to perform post-operative stitching. Those in intensive care administering drugs were doing something more complicated than an appendix operation.
News analysis, page 17