The UK's leading medical cannabis expert, Mike Barnes, has warned that it will be easier for private hospitals to prescribe and administer weed-based products, raising the possibility that access to the recently legalised drug will be two-tier.

Access to medicinal cannabis is likely to only be extended to an extremely small group of patients where there is an "unmet clinical need" not served by licensed drugs prescribed by the NHS, with many already struggling to satisfy the prescriptive criteria.

In private hospitals, however – where one-in-ten people are treated – access to the breakthrough drug could be easier, with profit-driven, better-funded operators able to appropriately care for patients within shorter timeframes.

"This whole system, sadly, lends itself in the short-term to private prescription in the independent sector, until the NHS gets its act together," said Barnes. "The recommendations of the RCP [Royal College of Physicians] and BPNA [British Paediatric Neurology Association] will probably stop prescription in the NHS, but not the private sector, because independent consultants are more likely to do what they think is in the best interests of the patient.

"It took three or four years for the medical establishment in Canada to feel comfortable with prescribing cannabis. It will certainly take a couple of years in the UK, and within that time there is an opportunity for the private sector to muscle in to the market – which is a pity, but a reality."

There is no clear set of rules about how to prescribe medical cannabis products, fuelling speculation that private hospitals – where doctors' decisions are subject to far less bureaucracy – can profit from confusion in the public sector.

It is estimated that more than a million people use cannabis regularly to treat a host of medical conditions, with a number of families being refused prescriptions for medicinal cannabis already.

One family told the BBC that they have been buying bottles of non-medical cannabis oil for £300 each to "ease [Harry's] potentially fatal epileptic seizures". However, other families cannot afford to spend thousands of pounds on medicines.