Dr Jonathan Harte, a Nottinghamshire GP, said the risks to safety grew as the day went on.
Describing his workload on the day of the snapshot survey, February 11, he said: "By lunchtime, I felt on the edge and risked missing urgent tasks and contacts, thus affecting patient safety."
Dr James Howarth, a GP in Spilsby, Lincs, dealt with 124 patients on the day of the poll.
"This creates patient safety risks," he said. "There are risks around having multiple patient notes open. We might forget consultant details, plans and actions, or prescribe for the wrong person, use the wrong labels on blood tests, and so on."
The survey found GPs work an average 11-hour day - eight hours of clinical care and three hours of administration.
The British Government has pledged to recruit 5000 more GPs by 2020, but has admitted that aim is unlikely to be met.
Professor Helen Stokes-Lampard, chairman of the Royal College of GPs said: "In my own practice recently, I had a 12-hour day and 100 patient contacts. GPs across the UK will tell similar stories."