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
A keeper killed by a tiger at a zoo in Cambridgeshire has been named in reports and described as the "shining light" of her workplace.
The two women charged with murdering the half-brother of North Korean dictator Kim Jong-un have appeared in court in Malaysia.
More than one million families will benefit from Labour's plan to overhaul "patchy" childcare provision, the party has said.