The National Training Surveys 2010 shows that, although most UK GP trainees rate their practical experience highly, GP trainers' responses suggest a decline in standards.
The survey found that 10 per cent of GP trainers are aware of trainees being signed off on their annual review of competence progression, record of in-training assessment or foundation training without reaching the appropriate standard.
A respondent to the survey said the 'tick-box nature' of the training programme increases the chance of borderline trainees 'slipping through'.
Another said the assessment tools used to rate students made it difficult to fail a trainee who is not meeting requirements.
The survey also said GP trainers felt 10 per cent of their trainees were not displaying the expected competencies for the level of training.
GP trainers' responses also suggest a decline in standards, with 50 per cent agreeing that trainees are less confident and less able to work independently than when they were trainees.
Meanwhile, the data show that 72 per cent of trainees rated their practical experience as good or excellent, and 78 per cent were fairly or very confident in acquiring competencies.
But the figures also show that 6 per cent (276 in total) of trainees who were coming to the end of their training believed they were not ready to take up a GP or consultant post.
The top area for under-preparedness was for planning and managing the service (198) followed by dealing with managers (151) and clinical (136).
GPC chairman Dr Laurence Buckman said it is 'very worrying' that trainers are signing off trainees when they do not have the required competencies.
He added: 'It is also true that trainees feel less competent than before, which is why the profession is calling for longer training.'
GMC chief executive Niall Dickson: 'We will not hesitate to step in where we believe trainee doctors are being asked to work beyond their competence or are not receiving the training they need.
'And we will continue to work closely with postgraduate deaneries and the NHS, to find effective solutions where there are specific serious concerns.'