The health regulator, the Care Quality Commission (CQC), has 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.
CQC carried out inspections at 1,000 GP practices in England and found examples of "very poor care" that put patients at risk.
"We found some surgeries where there were out of date vaccines in the fridge," Professor Steve Field, the CQC's new chief inspector of general practice said.
"You are talking about problems which can damage this generation and the next generation," he said.
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.
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Some GPs left private medical files laying around, had medicines that were out of date, filthy treatment rooms and employed staff who had not undergone criminal record checks.
Royal College of General Practitioners said many "GPs are working 11 hour days and seeing up to 60 patients a day in the face of increasing pressure to deliver safe patient care with diminishing resources."
RCGP's chair Dr Maureen Bake described a "funding black hole worth £9 billion" in England over the past eight years, which she said was a result of investment being shifted towards hospitals.
Dr Chaand Nagpaul, chairman of the British Medical Association's general practice committee, has said anecdotes of poor practice must not be used to distort reality.
"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," he said.