More than 40% of online healthcare services, such as those that provide GP consultations and prescriptions through independent websites and apps, are not providing "safe" care, the health watchdog has warned.
The Care Quality Commission (CQC) highlighted cases where independent providers were found to be prescribing high volumes of opioid-based medicines without talking to the patient's registered GP, and inappropriately prescribing antibiotics.
Other areas of concern included unsatisfactory approaches to safeguarding children and those who may not have the mental capacity to understand or consent to a consultation, and the inappropriate prescribing of medicines for long-term conditions.
There were also issues with providers not collecting patient information or sharing information with a patient's NHS GP, who should have an accurate and up to date record of their previous and current treatments and health problems.
The CQC, which has inspected every company that provides these services in England, said that as of February 28, 43% were found not to be providing "safe" care in accordance to the relevant regulations.
It added this was an improvement from the 86% on their first inspections.
It said it will continue to hold these providers to account until they become as safe as general practice in physical premises.
The watchdog also highlighted positive findings, including a company that provides sexual health services online.
It offers partner notification services where, with the patient's consent, it can confidentially trace their sexual contacts if they may have been at risk and encourage them to be tested.
The CQC found online consultations have the potential to improve access and convenience for some patients, such as those with physical impairments who find attending face-to-face appointments difficult, those with a sensory impairments, and those who live in rural areas and have poor transport links.
Professor Steve Field, CQC chief inspector of General Practice, said: "New methods of service delivery that increase access to care and give patients more control over how and when they see a GP have huge potential not only for patients but for the wider health system.
"However, while innovation should be encouraged, it must never come at the expense of quality. As with all health care services, patient safety must be at the heart of all decisions around what kind of care is offered and how it is delivered.
"This is why we have taken action where we have seen risks to patients - and why we have been encouraged to see many providers take note of our findings and make swift improvements to how they operate.
"This way of delivering primary care has an important place in the future of health provision - but it is still evolving."
Professor Helen Stokes-Lampard, chairwoman of the Royal College of GPs, said: "It's absolutely right that the Care Quality Commission holds organisations that provide online primary care services to the same high standards as any other healthcare provider.
"But it's very concerning to see that even now, 43% of online consultation providers have been deemed unsafe in some respect. New services will inevitably experience some teething problems, but when our patients' health is at risk urgent, swift action must be taken to comprehensively address these before the service is rolled out further.
"The inappropriate prescribing of antibiotics, for example, poses risks to individual patients but also is of great concern to the wider public - and the failure to collect and share a patient's data with their NHS GP could certainly have a detrimental effect on their future care."