Melanoma can be fatal
Potentially dangerous skin cancers are being treated by family doctors, despite guidance telling them to refer them to hospitals.
Surveys of GP skin cancer surgery found that even melanomas - the most lethal form of skin cancer - were being cut out at English surgeries.
The research found GPs often removed too little tissue, allowing cancer to return or delaying necessary treatment.
Extra training was needed, said the British Association of Dermatologists.
The majority of skin cancers are not dangerous if caught at a relatively early stage, and many can be removed safely at the GP surgery.
However, it is crucial that enough healthy tissue is "excised" around the edges of the cancer to stop the cancer growing back.
In 2006, the National Institute for Clinical Excellence issued guidance in England and Wales which said that appropriately trained GPs could remove most 'basal cell carcinomas', which are the most common and least threatening skin cancers.
However, it said that they needed to work as a team with hospital-based dermatologists, and make sure that if they suspected other types of skin cancer - including squamous cell carcinoma and malignant melanoma - they should refer them onwards.
Separate guidelines in Scotland also suggest urgent referral of melanoma and squamous cell carcinoma to hospital.
The studies presented at the British Association of Dermatologists Annual Meeting in Liverpool suggests that the NICE guidance is not being followed, and that GPs are not as good as hospital specialists at cutting cancers out.
In Derbyshire, an audit of tissue specimens sent for analysis after surgery revealed that nearly a third of skin cancers operated on by GPs had not been completely removed.
A separate study of skin cancer biopsies sent by GPs to a London teaching hospital showed that 14% of the tumours involved were "high risk", and should have been referred straight to a hospital specialist.
'Cancer could recur'
In Norfolk, analysis of the records of 80 patients with melanoma found that 13% of them had been incompletely excised or biopsied in primary care.
Similar problems were found with squamous cell carcinoma patients, many of whom ended up waiting far longer for the cancer to be fully removed compared with those referred immediately to hospital.
Dr David Shuttleworth, the clinical vice-president of the British Association of Dermatologists, called for extra training for GPs: "Incomplete excision of melanoma makes it more difficult to establish the true depth of the tumour, which in turn makes it trickier to plan the surgery and give a precise diagnosis.
"In the case of melanoma or squamous cell carcinoma, the cancer could recur if not fully removed.
"Incomplete excision of course also delays definitive treatment and causes additional worry to the patient."
Professor Steve Field, chairman of the Royal College of GPs, said: "This study highlights the need for GPs to follow NICE guidelines, particularly those regarding melanoma.
"The majority of GPs in this country are following these guidelines, and this should serve as a wake-up call for the rest."