Nearly a quarter of NHS walk-in centres have closed in the last three years, leaving an increasing number unable to access healthcare services.
The closures risk "increasing health inequalities", the NHS regulator Monitor has found, as workers, young adults and vulnerable people who struggle to access GP services no longer have easy access to a doctor.
Despite the popularity of the centres, since 2010 53 of England's 238 walk-in centres have closed, while more are under threat of closure, according to Monitor’s assessment of the service.
Walk-in services are particularly popular among young adults, women and poorer people, the preliminary report found.
People who use the service include those who find it difficult to access GP services such as the homeless, as well as groups with language or cultural barriers. Workers, students and people not registered at a GP practice also use the centres, the report said.
"The extended and weekend opening hours of walk-in centres, as well as the locations of some in city or town centres, allow those finding it difficult to take time off work to attend to primary care needs," the authors said.
"Overall, the evidence we collected suggests that walk-in centre closures, or possibly relocations/reconfigurations, can risk increasing health inequality if suitable alternatives are not put in place."
Monitor's executive director of co-operation and competition, Catherine Davies added: “First of all, there is an issue of access to primary care service - people are saying they find it difficult to get an appointment with a GP when they want to get it.
"The other issue that came through is that walk-in centres provide for vulnerable groups of people who are less likely to register with a GP.
"So if a commissioner is thinking about closing a walk-in centre, we're suggesting that they really need to think through how the health needs of these groups of people will be met."
Monitor found that the centres were closed for a number of reasons. Local health bosses sometimes believed the centres were generating "unwarranted" demand with many "worried well" attending.
In some cases the attendances exceeded expectations and the centres were shut because they were "too popular", officials said.
The regulator said some commissioners felt they were "paying for services twice" because they are compensating GPs to have patients registered at their practices and are also paying the centres for each attendee.
Shadow health secretary Andy Burnham said: "NHS walk-in centres are popular with patients and an important means of relieving pressure on A&E. When hospitals are under so much pressure, it makes no sense to close so many walk-in centres.”
Health Minister Lord Howe said: "Patients should be able to access good-quality out-of-hours NHS services without having to go to an A&E. Walk-in centres may be part of the answer but this isn't a one-size-fits-all solution. Family GPs, community services and pharmacists all have a part to play and it's good that Monitor is looking at how walk-in centres fit in.”