Dr Chaand Nagpaul of the British Medical Association said a national shortage of GPs was already a "real challenge" without the pressure to work additional hours.
Speaking after Prime Minister David Cameron urged longer opening hours for GPs' surgeries, Dr Nagpaul said they are "currently unable to cope with the increased demands of care".
He told ITV News: "These additional hours amount to around a 58% increase on current levels and we really don't have the GPs to meet current demands, let alone an addition 58% increase in hours."
More top news
The author, who explored feminist themes, was best known for her Earthsea books.
Jon Lansman denied reports that the group's supporters had drawn up a deselection hit-list of 50 centrists.
The film tells the story of a deaf girl who lives in a world of silence until a social worker teaches her the gift of communication.