The number of people waiting more than two weeks to see their GP in England has shot up by more than 1.5million this year, official data shows.
Between January and March, 12.3million appointments were made 15 days or more after patients had booked to see their doctor.
This is a 14 per cent rise from the 10.8million during the same period last year, and represents one in six patients overall.
Experts said the figures show how GPs' workloads are expanding as the number of appointments continues to grow but the number of doctors falls.
Waiting times vary across the country. The worst performing area kept almost one in three patients waiting a fortnight and the best fewer than one in 10.
Figures from NHS Digital showed patients in Swindon were most likely to wait 15 days or more for a doctor's appointment in March, when 30 per cent of them faced a two-week wait
The figures released today by NHS Digital suggest a shortage of GPs is continuing to take its toll on patients.
Doctors have repeatedly warned growing patient lists, ageing populations and reducing numbers of new recruits are damaging the system.
Although the vast majority of appointments are booked and completed on the same day – 10million in March – the number of long waits has risen.
Many appointments with long waits are routine follow-ups or non-urgent visits, but experts said the figures are still a sign of doctors' increasing workload.
Patients in Swindon are the most likely to wait 15 days or more between booking their appointment and actually having it, according to the data.
There, 30 per cent of the nearly 108,000 appointments in March took place more than two weeks after they were booked.
Other Clinical Commissioning Groups – local NHS boards – where at least a quarter waited two weeks included Newark and Sherwood in Nottinghamshire (27 per cent), and North Derbyshire, South West Lincolnshire, South Norfolk and the Isle of Wight, which all recorded 25 per cent.
Dr Richard Vautrey, chair of the British Medical Associaton's GP committee said: 'This data, while only providing a limited snapshot of the total work that GPs do, provides clear evidence of the rising workload pressure practices are under.
'We are providing a million more appointments each month, yet with hundreds fewer GPs.
Official figures showed in February that 41 per cent of GPs – around 10,000 doctors – are 50 or over and are expected to quit within the next five to ten years.
And 2.5 million patients are at risk of their local GP surgery closing because so many are relying on doctors who are close to retirement, it was last week revealed.
At the same time, fewer young doctors are choosing to specialise as GPs and are opting for other career paths as surgeons or specialists.
Many GPs are retiring in their 50s, moving abroad or leaving to work in the private sector, increasing the pressure on those who still work in the sector.
Appointment waiting times are getting longer and more people are going to A&E for minor illnesses because they can't see a doctor.
Despite an NHS a plan to recruit 5,000 extra GPs by 2021, numbers of family doctors are falling.
And 762 GP practices across the UK could close within the next five years, according to the Royal College of Nursing.
Rachel Power, chief executive of the Patients Association, last week told The Times: 'This is a desperate situation with potentially serious consequences for patients.'
'As recent BMA analysis showed, this winter saw the whole health service under immense pressure, from GP practices to hospitals and beyond.
'Doctors and healthcare staff across the NHS [struggled] to manage workload and keep up with mounting demand. The obvious knock-on effect is longer waits, which can cause understandable distress to patients, and equal frustration for doctors.'
In total, 4,262,723 patients waited more than 15 days for their appointment in March – 17.6 per cent of the total – with 1.09million of them waiting for more than four weeks.
The 15-day figure was NHS England's highest since November last year, when it was 5.14m – 19.7 per cent of the total.
But in March last year, after what was branded the health service's worst ever winter crisis, just 3,690,737 waited over two weeks – almost 572,000 fewer.
A spokesperson for NHS England said: 'Appointments booked and attended over 15 days include many patients who require routine follow-ups or prefer to book dates to suit them.
'Around half of all GP appointments are booked and taken on the same day, or within 24 hours.
'There is greater access to GPs now than ever before.'
Many of the best performing NHS groups were in London.
Only nine per cent of patients in City and Hackney waited more than 15 days, along with 10 per cent of those in Waltham Forest, Merton, and South Sefton and Knowsley in Liverpool.
West London, Lewisham, Redbridge, Lambeth and Croydon were all also among the 10 best performers.
Dr Vautrey added: 'While we know that many patients are waiting too long to be seen, this data shows that the largest proportion of appointments continue to be made and attended the same day, while the majority are seen within a week of booking.
'And though these figures show a rise in patients being seen more than two weeks after making their appointment, it is important to note that many of these will be appropriately booking ahead for return visits or regular check-ups.
'What these figures do not account for is the vast range of other activities GPs complete in their day-to-day work – including training, meetings and paperwork, which add significantly to their workload.
'The majority of evening and weekend appointments will also not have been included, as well as some home visits and telephone triage calls.'