The Medical Protection Society (MPS) said doctors have sought advice after being asked to sign such forms for patients taking part in running or other sporting events.
The MPS said GPs should be cautious and not feel under pressure to act outside the limits of their competence.
It said around 56% of the 250,000 applicants for the 2017 London marathon were first-time marathon runners. The growth in participation in such events meant GPs faced more fitness to participate requests, the MPS said.
GPs have also been asked to certify patients as fit for activities such as desert ‘fat camps’, mock hostage situations, or events where the patient must undergo psychological torture, the defence organisation said.
Dr Pallavi Bradshaw, senior medicolegal adviser at MPS, said GPs must be confident that they have sufficient knowledge of the patient and the nature of the event before deciding whether to sign such forms.
She said: ‘GPs may not always have the required expertise to deem the patient fit to take part without risk, and will understandably be worried about the implications of signing such a form.
‘GMC guidance requires a doctor to do their best to ensure reports they write are not misleading, and says they should not undertake assessments beyond their area of clinical competence.
But Dr Bradshaw said there were options where doctors can assist ‘within the limits of their competence’.
‘Depending on the wording of any declaration, signing the form with a qualifying statement may be appropriate. The GP would consider the information they have on the patient’s current and past medical history, which may be relevant to the event, and state that there are no known health conditions which render the patient unfit to participate.
‘Where a patient’s medical history is not straightforward or they are under the care of a specialist, the GP may wish to obtain advice first or refer the patient to a doctor with expertise in sports medicine.
‘There may be occasions where a doctor may decline to assist with completing the form - if it is in a language they do not understand for example. This can be the case when a patient wants to run a marathon in another country.
Dr Bradshaw advised GPs to contact their medical defence organisation for advice if they had concerns about any fitness to participate form or how to complete it.