The findings shed further light on the extent of the workforce crisis facing the profession, as the government struggles to deliver its pledge to increase GP numbers by 5,000 by 2020/21.
Six CCGs in England have more than 40% of their full-time equivalent (FTE) workforce aged over 55, and 20 - almost one in 10 of the total number across England - have a third or more GPs aged over 55.
With the average GP retirement age at 59, the figures - based on NHS Digital data published in August - suggest that parts of England are at severe risk of losing a huge proportion of their workforce within a handful of years.
The analysis of GP workforce data by GPonline comes less than a week after official statistics showed that the total FTE GP workforce had fallen by almost 1,300 over the two years to September 2017.
GPC chair Dr Richard Vautrey said the BMA had been warning about the 'demographic timebomb' facing general practice for years.
The government had acknowledged it had not invested enough in general practice, he said, but warned that new investment had not yet done enough to 'turn the tide'.
'We have been warning about the demographic timebomb for some time,' he said. 'There is a cohort of doctors who want to carry on working and if you enable them to focus on clinical care they can stay in workforce and provide a valuable service.
'But if that committed workforce is getting older there will come a time when they have to retire completely, and they are not being replaced by younger doctors.'
Dr Vautrey said that the older cohort would be larger if significant numbers of GPs had not already left the workforce.
He identified pensions, concerns about premises risk, the pressure and workload involved in CQC checks and revalidation as factors that had driven some older GPs out of general practice.
The overall decline in the GP workforce was 'really serious', Dr Vautrey warned, and reflected the huge pressure on general practice.
The heaviest concentrations of GPs over 55 are around London and south-east England, with a handful of pockets outside that part of the country where more than a third of GPs are in this age bracket. But the spread of areas where more than one in five GPs is over 55 is nationwide.
The health secretary repeated his commitment to the 5,000-GP target at the RCGP annual conference last month. Mr Hunt has previously admitted failing to prioritise building the GP workforce sufficiently during his more than five-year spell as health secretary, but responded angrily to a GP at the RCGP conference who asked why the profession should believe his claims that he was committed to tackling the crisis facing general practice.
The health secretary has highlighted an expansion in medical training places, salary supplements for GP trainees in underdoctored areas, increased GP contract funding over the past two years and plans for a state-backed indemnity deal as part of the drive to boost the workforce.