British doctors planning the world's first face transplant were warned the operation could go "horribly" wrong by leading surgeons.
The pioneering team must be aware of the potential risks of physical rejection and psychological distress to the patient, the Royal College of Surgeons said.
In a long-awaited report, a working party set up to look at the ethical and practical problems accepts the procedure will "inevitably" go ahead.
But it urges caution, saying the operation is not merely a "surgical exercise".
Professor Sir Peter Morris, chairman of the working party, said 15 minimal requirements must be met by any British team proposing to create medical history.
He said the greatest unknown was rejection of the new face, with recipients having an estimated 10 per cent risk of acute rejection within two or three months and up to 50 per cent chance of chronic rejection later on.
Sir Peter said it could be an insidious process which could lead to decreased mobility, redness and scaling because skin is "top of the league" when it comes to rejection in terms of tissues and organs.
A lifetime of immunosuppressant therapy also brings risks of infection and cancer, especially as patients get older, he added.
Working party member Professor Nichola Rumsey said the psychological effects of transplanting a face are "essentially unknown".
She raised concerns of the recipient ending up with a "mask-like" face, as humans use minute facial changes to express emotions.
And there was a "horrible" potential scenario if the donor skin was lost and the patient was left with a raw face and had to undergo conventional reconstruction.
Consultant surgeon Peter Butler was given permission last month to carry out a full face transplant by an ethical committee at the Royal Free Hospital, London.
He and his team have been approached by 34 patients wanting surgery but they have yet to choose a candidate.
The new report gives reluctant endorsement of the operation despite "considerable reservations" but only under strict criteria.
These govern competence of the team, counselling for the recipient and donor family, life-long support for patients and preparation for media attention.
Two partial face transplants have already been carried out in France and China and a team in America also have ethical permission to do a full transplant.
The Royal College's 56-page report published today represents a shift in its stance of outright opposition three years ago.
Sir Peter Morris said the position had changed, providing minimum criteria are met and 'as long as it is performed in a research setting".
He added: "It is not just a surgical exercise. The surgical expertise has been available for over 30 years."
Ethical committees make decisions independently of the RCS but the report will still have weight. Mr Butler has already welcomed the recommendations in the report and said his team met all the 15 requirements.