The NHS is seeing the first sustained fall in GP numbers in the UK for 50 years, the BBC can reveal.
An analysis by the Nuffield Trust think tank for the BBC shows the number of GPs per 100,000 people has fallen from nearly 65 in 2014 to 60 last year.
The last time numbers fell like this was in the late 1960s and it comes at a time when the population is ageing and demands on GPs are rising.
Patient groups said it was causing real difficulties in making appointments.
There have been reports of waits of up to seven weeks for a routine appointment, while those needing urgent appointments have been forced to queue outside practices in the early morning to guarantee to be seen.
The pressures on GPs are being looked at by the BBC as part of a special day of coverage, including a Panorama investigation.
Dr Helen Stokes-Lampard, president of the Royal College of GPs, said: "General practice cannot be allowed to fail. It is an absolute cornerstone of the NHS."
NHS bosses maintain that steps are being taken to improve access, with more GPs being trained and extra support staff recruited to work alongside them.
GP numbers on their way down
The Nuffield Trust analysis looked at the number of GPs working in the NHS - both full and part-time - per 100,000 people across the UK.
It shows that during the late 1960s the numbers were falling, before four decades of almost continuous growth.
A peak of 66.5 was reached in 2009, before the increases tailed off.
There have now been four consecutive years of falls with the biggest drops being seen in England.
The fall in GPs from 64.9 per 100,000 to 60 per 100,000 means the average doctor now has 125 more patients to look after than they did in 2014.
The Nuffield Trust believes another 3,500 GPs would be needed to get the NHS back to where it was in 2014.
There are just over 42,000 working currently, down by nearly 1,500 in four years.
What does this mean for patients?
Patients report it is getting more difficult to get an appointment.
The biggest survey of GP patients is carried out by Ipsos MORI for NHS England.
About 750,000 patients are surveyed each year.
While satisfaction ratings remain high, only two in three people needing an urgent appointment on the day are able to get one.
Kent is one of the areas with the worst problems.
The local patient network - Healthwatch Kent - has been investigating access to GPs.
Its chief officer Steve Inett said: "Patients have been reporting they are having to wait six or seven weeks for routine appointments. And even when they try to get an urgent one they can be forced to queue outside surgeries before they are open.
"It is a real problem. Some GP surgeries are really struggling to recruit the staff they need, particularly the smaller surgeries."
Where are all the GPs going?
For a number of years the NHS was struggling to attract junior doctors to become GPs.
At one point, as many as one in 10 training places was going unfilled.
That has now been rectified - and the number of training places increased.
Last year, nearly 3,500 GP trainee posts were taken up in England, up by 800 since 2014.
This boost in numbers has yet to be fully felt as it takes at least three years to train a junior doctor to become a GP.
What is more, one in three junior doctors who accept places on GP training courses, drops out of the system, according to the Nuffield Trust.
Meanwhile, the numbers retiring early have been increasing.
Two-thirds of retirements by GPs come early - double the rate seen just five years ago.
The BMA said doctors were being asked to work longer and harder, without recognition or an increase in pay.
Dr Richard Vautrey, of the BMA, said workloads were now "unmanageable" for many.
It's not safe - GP
Dr Carmel Boyham Irvine has worked at the North Road West Medical Centre in Plymouth for 26 years.
She has seen demand increase as the population ages with patients needing help and care for ever more complex conditions.
She now deals with 50 to 60 patients a day - either on the phone or in person.
She told BBC's Panorama programme the situation was simply "not safe".
She is one of two GP partners - senior doctors who are responsible for the practice.
They have been trying to recruit another partner for five years but have had no success.
During that time the size of the practice's patient list has increased by 1,300 to 8,300.
"The sustainability of this particular practice is questionable," she added.
How the NHS is tackling the issue
All parts of the UK said steps were being taken to address pressures on general practice.
As well as increasing training places, steps are also being taken to increase the number of health staff to work alongside GPs.
More than 20,000 extra physiotherapists, pharmacists, paramedics and support workers are being recruited to help see GP patients in England.
Dr Nikita Kanani, of NHS England, suggested this would help ease demand on GPs.
But she acknowledged the service was facing "significant pressure".
Similar plans are being introduced in Scotland and Wales with a Welsh government spokesman saying the aim is to "transform" the way services are organised.
Panorama's GPs, Why Can't I Get An Appointment, is on BBC One at 19:30 BST on 8 May