Most NHS trusts are contractually obliged to send a letter to practices within 48 hours of an outpatient appointment, a target that falls to 24 hours in April.
After analysing around 40,000 letters dating back to April 2008, Dr Peter Crouch, of the Taw Hill Medical Practice, found just 2 per cent arrived within two days.
Just 6% arrived within three days, and one letter taking 344 days to arrive.
The average delay was between 11 and 16 days, the analysis found.
Dr Crouch said many of his consultations are wasted as patients cannot be seen without test results or details of prescription details.
‘17,000 delayed letters is 17,000 bits of information that I could have used to help patients,' said Dr Crouch.
‘I had four patients in during extended hours last week. Unfortunately they had to reschedule because I hadn't received a letter detailing their treatment in hospital.'
GPC negotiator Dr Beth McCarron-Nash said the evidence ‘set alarm bells ringing'.
‘If it's happening in one practice, it is probably happening elsewhere and that should be investigated,' she said.
‘I can't speak nationally but certainly in the south west our consultant colleagues are under immense pressure and the speed at which they issue letters is variable.
‘They need to be legible and fit for purpose. It's not just about how quick they come but also how accurate and complete they are.'
Conducting the audit was ‘easier than I had ever imagined' said Dr Crouch, who realised his scanning system had created a powerful data set with details for most hospital departments, going back years.
Dr Crouch urged other practices to investigate trends in trusts' performance. He hopes his data will help NHS Swindon put pressure on hospitals.