Londoners can now book GP appointments for evenings, weekends and bank holidays.
An extra 75,000 routine appointments a month will be available in the capital as part of a scheme by NHS England.
The £26m initiative aims to reduce pressures on stretched A&E departments.
The British Medical Association (BMA) was "encouraged" that the plans cover extra staffing expenses, but was worried by doctors' rising workloads.
London is the first area to offer the extended service.
If surgeries are fully booked or closed when registered patients call up, they will be able to book with other nearby practices.
GPs will have access to electronic patient records so that patients do not have to be seen by their regular doctor.
Other recent attempts to increase efficiency in the NHS include GP consultations via video link.
NHS England's Prof Jane Cummings said: "Appointments at evenings and weekends will help manage pressures on urgent and emergency care services, especially over the busy winter period."
Dr Richard Vautrey, from the BMA, said: "There is still an urgent need to address the significant pressure being caused from rising workload, tightening budgets and staff shortages, which are pushing many GP services to breaking point."
Andrew McCracken, from National Voices, said the increased access was beneficial for patients and improves efficiency.
But he added that recruiting enough healthcare professionals to cover the extra hours was a "longer term issue".