Almost a quarter of NHS walk-in centres have closed in just three years - leaving patients unable to access the care they need, a health regulator has warned.
The centre closures risk "increasing health inequalities" among patients who have difficulty accessing GP services, including workers unable to take time off to see their family doctors, Monitor said.
Despite the popularity of the centres, since 2010 53 of England's 238 walk-in centres have closed, according to the regulator's review into the service.
And more are under threat of closure.
Monitor has suggested that, in areas where the centres have closed, patients may not be able to access the care they need.
The centres are particularly popular among young adults, women and poorer people, according to the preliminary report.
In addition, people who use the service include those who find it difficult to access GP services such as the homeless and groups facing language and cultural barriers.
Workers, students and people not registered at a GP practice also use the centres.
"Accessing traditional GP practices often requires people to take time off work, yet this can be difficult or simply not possible for some," the report states.
"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, including seeking preventative services and routine checks for chronic conditions."
The authors added: "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 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.
But a patient poll carried out among 2,000 walk-in centre users found that just 8% would treat themselves at home if the service was not available, 21% would go to A&E and a third would visit their GPs.
In some cases the attendances exceeded expectations and the centres were shut because they were "too popular", officials said.
Monitor's inquiries lead, Sondra Roberto, said: "In some cases they were having increased demand and that led commissioners to look at them and close them because they were too popular.
"Sometimes a concern among commissioners is that people are coming in for very minor conditions, and they would rather not spend the resources on things like coughs and colds. But there is a wide variety of things people come in for."
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.
It suggested that the payment system might need to be reformed to fix this situation.
Monitor's executive director of co-operation and competition, Catherine Davies, said: "We tried to understand a bit about the effect of the closures of walk-in centres. 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. Walk-in centres therefore provide an alternative to those people who do have difficulty getting an appointment with their GP.
"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."
The centres - which treat minor illnesses and injuries without an appointment - were a flagship policy of the previous Labour government.
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.
"People need services and support seven days a week and that is why the last Government introduced NHS Direct and walk-in centres. The decision to dismantle them is one of this Government's worst acts of vandalism and patients are paying the price."
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.
"We're working with NHS England to ensure services are tailored to the local needs of patients and we've agreed a £3.8 billion integration fund to help join up health and care services."