The latest surge in registrations - which follows an advertising campaign - gives GP at Hand the 21st-largest list size in England.
Evidence that the service is continuing to attract patients away from traditional GP practices comes just a week after the NHS long-term plan promised patients a 'right' to have access to digital GP consultations over the next five years.
Practices look set to be in competition with 'digital first' models such as GP at Hand, with the long-term plan promising patients access to digital consultations either 'from their own practice or, if they prefer, from one of the new digital providers'.
Growing patient list
The number of patients registered at south-west London's Lillie Road Medical Centre, which hosts GP at Hand, is now 41,690 - 17 times higher than it was in April 2017, analysis of NHS Digital data by GPonline reveals.
Since November 2017, when the service was made available to out-of-area patients who live or work across a broad area of greater London, the number of patients registered with the service has increased by more than eight times.
A staggering 87% of patients added to the service's list since November 2017 are aged between 20 and 39 years old - contributing to claims that GP at Hand is 'cherry picking' younger patients, denying traditional GPs funding and leaving them with more complex cases.
Overall, there are now 20,647 patients aged 20-29 years old registered with the service, with a further 14,349 aged 30-39. Just 7% of GP at Hand's total patient list is aged outside of the 20-49 age range.
Babylon did not immediately reply to a request for comment, but has repeatedly denied that the service is cherry picking young patients. It has also said that 'patients and GPs are flocking to GP at Hand because they recognise the potential that high quality, digital-first 24/7 NHS GP services bring'.
The service has also faced criticism over limits on who can register. Hammersmith and Fulham CCG documents confirmed last November that a list of restrictions on who can register with GP at Hand would be removed from the service's website, although patients 'will still be advised that sometimes it may not be clinically appropriate for them to register with a practice that is not local to their home'.
Health minister Matt Hancock said last year that the funding model for general practice could be overhauled to accommodate the rise of services such as GP at Hand.
GP at Hand was censured by the advertising watchdog last year for misleading claims in a campaign that ran from November 2017 to February 2018.