The first national inspection of more than 900 GP surgeries in England has found one in three is failing to meet basic standards.
The Care Quality Commission unearthed failings in some practices, many of which had been selected after concerns.
It said it had found examples of poor standards in the handling of medicines and cleanliness, with maggots found at two surgeries.
Overall, concerns were expressed about a third of practices.
In nine cases the failings were so serious that they could "potentially affect thousands of people", the CQC said.
These practices have been ordered to improve, although in one case new GP management has been brought in.
The checks were targeted mainly at those practices deemed to be high risk ahead of next year's rollout of the first national GP inspection regime.
Among the problems identified across the board were concerns about the way medicines were managed.
The CQC said it had found examples of emergency drugs being out of date and vaccines not kept in properly regulated fridges - something which could damage the effectiveness of the jab and cause outbreaks of disease.
The regulator said some practices were "visibly dirty", and in one of the better-performing practices, Dale Surgery in Sneinton, Nottinghamshire, inspectors found maggots. The details of a second practice where maggots were found have not yet been released.
Dale Surgery said it took action to deal with a "small number of insect larvae" in a hallway, adding that a pest control company confirmed there was no evidence of wider infestation.
"We are responding to the actions we have been asked to take," it said in a statement.
In another case, a practice was found to be carrying out patient consultations in a room with no door.
It also said practices were not always doing the necessary employment checks, and access to routine appointments was also a "constant theme", with some patients reporting they were unable to get one for weeks.
The checks were carried out to help test how the GP inspection regime will work when it is launched next April as well as to inspect practices where concerns had been flagged up to the regulator.
Of the 910 inspected, 80% were targeted because of known concerns. The remainder were chosen at random.
The CQC is aiming to inspect all 8,000 practices over the following two years and give them one of four ratings - "outstanding", "good", "requires improvement" and "inadequate" - as is happening with hospitals.
The move will mark the first national independent inspection of practices - to date GPs have faced only local audits at most.
Prof Steve Field, the CQC's new chief inspector of GPs, said it would herald the end of an era where poor care was tolerated in general practice.
He said the problems highlighted in the checks had sometimes been known about locally for years.
"We are hearing about problems that people are very worried about but no-one has tackled in the past.
"We need to make sure that everyone, from the most well-off to the most disadvantaged, can get access to really good care."
He told BBC Radio 4's Today programme: "Of the really poor practices we're talking nine or ten out of that 900, so very tiny numbers. But, if you have a problem with, for example, storage of vaccinations for children, the impact could be on hundreds of patients. So the numbers are small, but the impact is large."
Roger Goss, of the campaign group Patient Concern, said: "The failings are inexcusable. The standards the CQC wants met are the basic minimum for protecting patients' safety, let alone improving their health."
Health Secretary Jeremy Hunt added: "Patients have a right to expect the best care from their GP practice."
Dr Maureen Baker, who chairs the Royal College of GPs, said: "Breaches of procedure cannot be condoned - even if they are isolated incidents."
But she added: "Patients should expect high quality and consistent care from their GP practice, and the vast majority of practices do an excellent job of delivering quality care while maintaining the highest possible standards."
• The practices which were ordered to improve were: Dr Harira Syed's practice, Rochdale, Lancashire; Norris Road Surgery, Sale, Cheshire; Wolds Practice, Horncastle, Lincolnshire; Kingshurst Medical, Birmingham; Lincoln Road Surgery, Peterborough, Cambridgeshire; Northfield Surgery, Doncaster, South Yorkshire; Dr Satish Patel's, Reading, Berkshire; Long Street Surgery, Wigston, Leicestershire; King Street Surgery, Burton upon Trent, Staffordshire.