Patients could be at risk of harm from online companies offering doctor services, the Care Quality Commission (CQC) has warned.
It comes after investigators found two firms were putting patients at risk by failing to take down their medical history before prescribing medicines.
There were also question marks over whether clinicians had the relevant skills or qualifications to diagnose illnesses or prescribe.
Inspectors also found no processes for contacting a patient's GP, including when medication was prescribed that required monitoring or follow-up.
Medics also failed to check a patient's identity before prescribing online, and there was no way of checking if they lacked capacity to consent to the treatment.
The two online firms were MD Direct (which had traded through the website assetchemist.co.uk) and HR Healthcare Ltd (through the website treated.com).
The CQC said its inspection of HR Healthcare Ltd was influenced by an investigation by BBC Radio 5 Live, which looked at its online sale of antibiotics.
Following this, the CQC suspended the registration of HR Healthcare Ltd while MD Direct voluntarily cancelled its registration.
The site assetchemist.co.uk now uses a different online GP provider for its prescription service.
As well as these two inspections, a review of all 43 online services that are registered with the CQC showed some others were potentially posing a risk to patients. The CQC has now brought forward a programme of inspections prioritising those.
It is particularly concerned about sites where patients can choose their own drug and select their symptoms or diagnosis from a drop-down menu.
The prescription is then reviewed by a medic and passed to a pharmacist who processes it.
Professor Steve Field, chief inspector of general practice at the CQC, said: "The growth in online technology presents a real opportunity to improve people's access to medical advice and treatment. It is important that healthcare services continue to innovate.
"However, in some cases we have found websites which in effect allow people to select their own medication, including medicines restricted as prescription-only, with little or limited clinical oversight.
"Patients can go online, self-diagnose their condition, order their own medicine and obtain a prescription from the online doctor service, with minimal checks on who they say they are and whether the medication is safe or appropriate for them, often within a matter of seconds.
"We know there are often inadequate identity checks, no checks on patient history or suitability, no checks with patients' GPs, and no follow-ups or monitoring.
"Following our review of all online services registered with CQC, we will now visit each provider, working closely in partnership with the relevant regulators, and checking that providers are following the appropriate professional guidance.
"We will take action to cancel or suspend the registration of providers who are putting their patients at risk."
Professor Field said that, as with conventional GP surgeries, online companies and pharmacies are required to provide safe, high-quality and compassionate care.
"They must not cut corners," he added.
A further joint statement from four regulatory bodies - the CQC, the General Medical Council, the General Pharmaceutical Council and the Medicines and Healthcare products Regulatory Agency (MHRA) - reminds firms and their medics that they must provide safe and effective care, including following professional guidelines.
Health Minister Lord O'Shaughnessy said: "We have empowered the CQC to run a tough and comprehensive inspection regime and commend their work to uncover failings in digital care provision.
"Online providers can be a convenient option, but patient safety must always be the priority and we urge the public to follow CQC's advice when buying medicines online."
Gerald Heddell, director of inspection, enforcement and standards at the MHRA, said: "Prescription-only medicines are prescription only for a reason and should only be taken under the supervision of a healthcare professional.
"A proper consultation with a medical professional is essential to ensure that an appropriate diagnosis of your condition can be made, your medical history can be reviewed, your recovery can be monitored and any adverse reactions can be dealt with."
Jane Mordue, chairwoman of Healthwatch England, said: "The internet has changed the way we do almost everything, from booking holidays to banking.
"But when it comes to our health it is even more important to ensure people know that the online services they are accessing are safe."