Inspectors have uncovered a catalogue of failings at some GP practices, with medicines stored in a way that puts children and patients at risk of infection and rooms so dirty they had maggots.
The Care Quality Commission (CQC) health regulator carried out inspections at 1,000 practices across England and found examples of "very poor care" that put patients at risk.
While many people received an excellent service, a third of surgeries (34%) failed to meet at least one of the required standards on good practice and protecting patients.
In nine practices "there were very serious failings that could potentially affect thousands of people", the CQC said, and in 90 practices follow-up inspections had to be ordered to ensure improvements were made.
Some GPs left private medical files laying around, had medicines that were out of date, filthy treatment rooms and employed staff who had not undergone criminal record checks.
In one of the better-performing practices, inspectors found maggots and dirty conditions, while in another consulting rooms had no doors and people could hear what was being said to the GP.
In some surgeries, emergency drugs were out of date and fridges were not always checked to ensure they were at the right temperature.
The CQC said this puts children in particular at risk because failure to store vaccines at the right temperature can reduce their effectiveness, leading to an outbreak of a contagious childhood disease such as measles.
The reports come as Professor Steve Field, the CQC's new chief inspector of general practice, set out his new approach for the inspection and regulation of GPs and GP out-of-hours services.
" We found some surgeries where there were out of date vaccines in the fridge," he said, adding that people who wrongly think they are immune could become " very, very poorly and then die".
He said a woman who thought she was immune to German measles due to vaccination could potentially give birth to a deaf and blind baby.
"You are talking about problems which can damage this generation and the next generation," he said.
Prof Field said patients across the board had difficulties getting appointments.
In one Birmingham practice, people were queuing outside in order to make an appointment, he added.
At one practice, both GPs had referred each other to the GMC for incompetence. They were no longer working in the practice.
In another practice in Leeds, the inspectors turned up but there were no GPs.
At Dale Surgery in Sneinton in Nottinghamshire, inspectors found maggots and other insects, as well as dust and cobwebs.
The surgery immediately sorted out the problem but inspectors said there was "no regular, effective and on-going monitoring of these standards".
Prof Field said: "We can talk about the fact we found maggots in a treatment room.
"And when we asked the question - and this is a good practice - the nurse said yes we do seem to have a bit of a problem.
"But they sorted it straight away."
Prof Field said there was a widespread belief that out-of-hours GP care was "risky" because GPs don't have access to medical records and do not know the patient.
Prof Field's new-style inspections will start in April 2014 and will involve a CQC inspector, a GP, a practice nurse or practice manager and a trainee GP.
Inspectors will visit every clinical commissioning group area in the England area once every six months, inspecting a quarter of the practices in that area.
Every practice will have been inspected by April 2016 and given Ofsted-style ratings.
From January, new inspections will also focus on GP out-of-hours services.
Prof Field said: "We need to make sure that everyone, from the most well-off to the most disadvantaged, can get access to really good primary medical care; this is something which I intend to champion as chief inspector.
"When something goes wrong in general practice, it has the potential to affect thousands of local people.
"GPs don't work in isolation, so we will also be considering the quality of communication between out-of-hours care and other local services, including GP practices, care homes and emergency services."
Health Secretary Jeremy Hunt said: "Patients have a right to expect the best care from their GP practice. That's why we have introduced this new, tougher system of inspection which will root out poor standards and celebrate the best. "
Some 37% of GP practices inspected on the standard relating to their workers failed to meet it, while 24% of practices inspected on the standard relating to safety and suitability of premises also failed.
Some 23% of practices inspected on their management of medicines failed the standard, while 22% failed the standard on cleanliness and infection control.
But just 5% failed a standard on the care and welfare of people who use the services and only 3% failed on respecting and involving people who use services.
Dr Maureen Baker, chairwoman of the Royal College of GPs, said: "Breaches of procedure cannot be condoned - even if they are isolated incidents - but the inspections were largely targeted at particular practices which had already been identified as having problems.
"Specific areas for improvement have been found and the CQC must now work with these practices to ensure that they meet the necessary standards and that other practices can learn from their experiences."
She said GPs are currently facing increasing pressure to deliver safe patient care with diminishing resources.
"GPs carry out 90% of all NHS patient contacts, yet a funding black hole worth £9bn has emerged for general practice in England over the last eight years as a result of investment being shifted towards hospitals.
"Family doctors now receive only 8.39% of the overall UK NHS budget - yet our workloads are spiralling as the population ages and more patients are presenting with multiple and complex diseases.
"Many GPs are now routinely working 11 hour days and conducting up to 60 patient consultations in a single day.
"Half of GPs believe they can no longer deliver safe patient care due to increasing pressures and unsustainable workloads."
Dr Chaand Nagpaul, chairman of the British Medical Association's general practice committee, said: "While most patients receive high-quality care from their GP, we need to understand where and why shortcomings in a small number of practices exist, and the BMA is committed to working with the chief inspector to improve standards.
"It's important, however, that anecdotes of poor practice are not used to distort the reality, which is that the overwhelming majority of hard-working GPs provide high-quality care which is appreciated by patients."