- 9 updates
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."
The Royal College of General Practitioners (RCGP) said some GPs are already routinely working 11-hour days and cannot do more "without the funding and resources to back it up".
RCGP chairwoman Dr Clare Gerada said: "GPs are keen to do more for their patients - with many already working 8-8 and at weekends - but we are heaving under the pressure of ever increasing workloads and diminishing resources, including a chronic shortfall of GPs.
"General practice currently counts for 90% of patient contacts within the NHS yet receives only 9% of the budget.
"We now need the Government to go much further and give general practice its fair share of the NHS budget so that GPs can deliver more care and better access to services for their patients in the community."
The Conservatives have to "win the trust of the British people" if they are going to remain in power after the 2015 General Election, David Cameron has said.
Speaking after a Daybreak poll showed over half of voters no longer trusted him, the Prime Minister defended his record in Government, saying the Coalition had made difficult but necessary cuts.
However, he also admitted the austerity programme "had not been easy for people".
Prime Minister David Cameron told Daybreak the pilot scheme to extend the opening hours of GPs' surgeries is "a very good step forward."
He continued, "It will also help hard-working people who often want to go to see a GP but find it difficult to get off work in order to do so."
Health Secretary Jeremy Hunt has called on GPs "to find new ways of working" in a bid to create greater flexibility for patients.
Mr Hunt, who will be speaking about the government's GP initiative at the Conservative Party conference today, said: "We live in a 24/7 society, and we need GPs to find new ways of working so they can offer appointments at times that suit hard-working people.
"Cutting-edge GP practices here in Manchester are leading the way, and we want many more patients across the country to benefit."
David Cameron's £50 million trial to encourage GPs' surgeries to open for longer will also look at improving access to doctors.
The extra cash from the trial will be offered to groups of GPs proposing the most effective ways to improve access.
As well as extended surgery hours, ministers hope the trial will encourage a more effective use of technology - such as carrying out consultations with patients via video calls, email and phone.
Online appointment booking, electronic prescriptions and allowing people to visit a number of different surgeries across an area are also measures being sought from the first wave.
The first pilot projects are due to be operating by April 2014 with the hope they will be copied widely across the country.
David Cameron has called for "greater flexibility" from GPs, which could see patients contact their doctor on the phone, through an email or on Skype.
GPs' surgeries should be open seven days a week from 8am until 8pm, David Cameron said as he announced a £50 million trial to encourage longer opening hours.
Around half a million patients are expected to be covered by a pilot project in parts of England as the government seeks to cut pressure on A&E departments.
A recent NHS survey found that almost one in five patients said inconvenient appointments were a concern and 70% backed weekend and after office hours opening.