The finding comes as GMC guidance on child protection, published in July, comes into effect.
A total of 86% of respondents to a poll carried out by the website Netmums on behalf of the GMC said doctors should report concerns even if they could not prove them.
Almost all respondents to the poll – 99% – said doctors should take steps to find out whether a child was at risk if their parents were taking illegal drugs or abusing alcohol.
A total of 94% of respondents said doctors had a responsibility to find out if a child was at risk even if they were treating only adult patients.
The findings support recommendations in the GMC guidance.
Niall Dickson, chief executive of the GMC, said: ‘Child protection is a difficult area of practice, complicated by uncertainty and often very emotionally challenging.
'Parents and carers need to have full confidence that if there are any issues raised about the safety of their child, their doctor will take the right course of action.
'Part and parcel of this is making sure that doctors communicate properly with both parents and children to convey any concerns they may have.
'Our new guidance will help guide doctors toward making the correct decisions in this challenging but essential area of work.’
Child protection officer for the Royal College of Paediatrics and Child Health, Dr Amanda Thomas, said: ‘When a doctor suspects a child may be the subject of abuse, the prospect of communicating this to parents can be daunting.
‘This new guidance should give thousands of doctors the confidence to raise these concerns and help communicate them in the best possible way to parents.’