Figures released by NHS Digital show that GP at Hand has seen the first drop in its list size since it opened up to out-of-area patients in November.
The number of patients registered with the Lillie Road Medical Centre, where patients sign up for GP at Hand, fell to 23,997 on 1 April - down from 24,652 a month earlier.
Of the 655-patient reduction in the practice's registered list, 475 have been lost among patients aged between 20 and 39 - the groups in which numbers had been rising fastest. A spokesman for the service said it had temporarily slowed down registrations to 'make sure our systems...are well able to manage expected future growth'.
The reduction in patients registered with the service comes as a surprise given a surge in registrations reported at the start of March. GPonline reported last month that more than 5,500 new patients had signed up with GP at Hand between 1 February and 1 March, in its fastest one-month rise since 2017.
The Fulham-based Lillie Road Medical Centre's total list size has grown from 4,970 to 23,997 in the four months since it offered access to the GP at Hand video consultation service to people outside its catchment area. Anyone living in a broad area of London or working in central London may be eligible join the service, which requires them to quit their existing practice and register as an out-of-area patient.
A total of 85% of new registrations since November have been from patients aged between 20 and 39 years old, official data from NHS Digital reveal. Of all new patients signing up since GP at Hand was opened up to out-of-area patients, 94% are aged between 20 and 49.
GP leaders have warned that the predominantly young profile of patients joining the service and rules that restrict access for people with complex needs show the service is 'cherry picking' younger, healthier patients.
NHS managers have also warned that the rapid growth of the service risks destabilising other practices, and leaving the local health service with a deficit. Hammersmith and Fulham CCG has said the growth of the service could drive up costs by more than £10m and push it into deficit during 2018/19.
GPC chair Dr Richard Vautrey told GPonline: 'Whatever the list size, our underlying concerns remain about the inappropriate use of out-of-area registration arrangements and the way this undermines the registered list on which general practice and ultimately the NHS as a whole is based.'
A spokesman for Babylon, the company that provides the technology behind GP at Hand, said: 'In March we temporarily reduced the rate at which we registered new members to make sure our systems - particularly on the NHS IT side - are well-able to manage the expected future growth well.
'NHS commissioners are fully aware of this approach, and registrations have already re-started. With over 300 people registered on Monday alone, we expect April to be another busy month. With people moving in and out of London all the time, there will always be some de-registrations in any given month, and the net effect for March is the very small reduction in list size.'