Thousands of patients are being denied prescriptions for cannabis despite the law change because of "botched and cruel" medical guidance, MPs have warned.
Conservative former minister Sir Mike Penning was among those on Monday to say the material being relied on by the NHS is shutting down the fresh law.
Billy Caldwell and Alfie Dingley, two severely epileptic boys whose conditions helped change the laws, would not be able to get renewed access to their medication under the guidelines, their families say.
Sir Mike (pictured) hit out at the Royal College of Physicians (RCP) and British Paediatric Neurology Association (BPNA) guidance as "crushing the hopes of many thousands of patients".
"We are now in the quite frankly cruel and ludicrous position of families with severely epileptic children once again having to fundraise to go abroad to get access to a medicine that we have just legalised in the UK," he said.
"Those responsible for this botched and cruel outcome should hang their heads in shame."
Labour's Tonia Antoniazzi, who co-chairs the all party parliamentary group on medicinal cannabis with Sir Mike, said she was "outraged and dumbfounded in equal measure" by the restrictions, which were introduced on Wednesday.
Medicinal cannabis expert Professor Mike Barnes said specialists will not prescribe the drugs because of RCP advice stating there is no strong evidence cannabis can help with chronic pain.
And, he added, the BPNA advising non-licensed cannabis products should only be prescribed when surgery is not a possibility.
RCP president Professor Andrew Goddard defended the guidance, saying there is not enough evidence to support the drugs' use for pain.
"We would welcome high quality studies into the use of cannabis-based medicinal products for pain," he added.
Among those to be restricted is nine-year-old Teagan Appleby, who is being treated for severe epilepsy in Evelina London Children's Hospital.
Her mother, Emma Appleby, of Canterbury, criticised the "chaotic and cruel" approach and said her daughter's medication has been withheld by the trust, citing the drug's regulatory status.
Seven-year-old Alfie currently has a special licence granted from before the law change, but his mother Hannah Deacon fears the guidance would rule out a renewal.
"We were told the law had changed because of the work we and Billy Caldwell had done, but we are worried it won't benefit us," said Ms Deacon, of Kenilworth, Warwickshire.
Co Tyrone's Charlotte Caldwell, whose 13-year-old son is being treated with cannabis oil, has said he would not be able to get a repeat prescription.
An NHS spokesperson said the law change means specialists can prescribe the products for a "small number" of patients, where their needs are not met and there is evidence of benefit.
The decision to reschedule the cannabis products came following a specially commissioned review, with chief medical officer Dame Sally Davies concluding there is evidence they can have therapeutic benefits.
However, the drugs can only be prescribed by a specialist doctor, not a GP, on a case-by-case basis.
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