Medicinal cannabis will be made available on prescription after it was approved for use by the government.
Doctors will be able to prescribe medicine derived from marijuana “by the autumn”, the Home Office announced.
The home secretary, Sajid Javid, said: “Recent cases involving sick children made it clear to me that our position on cannabis-related medicinal products was not satisfactory.
“Following advice from two sets of independent advisors, I have taken the decision to reschedule cannabis-derived medicinal products – meaning they will be available on prescription.
“This will help patients with an exceptional clinical need.”
But Mr Javid said it was “in no way a first step to the legalisation of cannabis for recreational use”.
Possession of the class B drug will still carry an unlimited fine and up to five years in jail, while dealers face 14 years.
It comes after Billy Caldwell, a 12-year-old boy who suffers from a rare form of epilepsy, was granted an emergency licence allowing him to be treated with medicinal marijuana by Northern Ireland’s Department of Health earlier this month.
Billy had previously been granted a short-term licence by the Home Office to allow him access to cannabis oil, which his mother, Charlotte, says helps to control his seizures.
Ms Caldwell said the rescheduling of medicinal cannabis was “incredible”. She said: “For the first time in months I’m almost lost for words, other than ‘thank you Sajid Javid’.
“That the home secretary has announced that medicinal cannabis can now be prescribed by GPs on Billy’s 13th birthday is amazing. I wonder if he knew? Never has Billy received a better birthday present, and never from somebody so unexpected.
An initial review by Dame Sally Davies, chief medical adviser, concluded there is evidence medicinal cannabis has therapeutic benefits.
The Advisory Council on the Misuse of Drugs (ACMD), which carried out the second part of the review, last week said doctors should be able to prescribe medicinal cannabis provided products meet safety standards.
It recommended cannabis-derived medicinal products should be placed in Schedule 2 of the Misuse of Drugs Regulations 2001.
Cannabis was previously classed as a Schedule 1 drug, meaning it was thought to have no therapeutic value but can be used for the purposes of research with a Home Office licence.
The Department of Health and Social Care (DHSC) and the Medicines and Healthcare Products Regulatory Authority (MHRA) will now develop a clear definition of what constitutes a cannabis-derived medicinal product, the Home Office said.
They will then be rescheduled and made available on prescription.
Karen Gray, whose son Murray suffers from epilepsy and has been treated with medicinal cannabis, said the change represented a victory for those who sought access to the treatment.
She told The Independent: “It’s really going to help children out there, so it’s brilliant news. There are a lot of kids out there who aren’t getting this help and hopefully now they will.”
Ms Gray started a 38 Degrees petitionwhich garnered 240,000 signatures to make medicinal cannabis available on the NHS.
Sir Mike Penning, cochairman of the recently established cross-party parliamentary group on Medical Cannabis Under Prescription, said: “I have just spoken to the home secretary to say how pleased I am that he has acted so quickly and I pay testament to the families and campaigners that have fought so hard to achieve this dramatic change in policy.
“This announcement brings hope to many thousands of people.
“However, there is still a very important body of work to define exactly which products will be allowed and how they will be regulated.”
He added: “Any move to restrict medical cannabis in the UK to a very narrow range of derived products, each requiring full pharmaceutical trials, thereby blocking out the many products available overseas, will lead to great disappointment and be a missed opportunity.”
Before the changes are brought in, clinicians can still apply to the independent expert panel on behalf of patients who want access to medicinal cannabis products.
The home secretary said licence fees for applications made to the panel will be waived, and those already granted will not be charged.
Genevieve Edwards, director of external affairs at the MS Society, said: “This is exceptional news and we want to thank the home secretary for the speed at which this decision has been made.
“We started campaigning for cannabis for MS exactly a year ago and it’s incredible to see how far we’ve come since then. The priority now has to be making sure everyone who could benefit can access cannabis in a safe and responsible way.
“We plan to work closely with the government to determine what exactly this will mean for people with MS. This life-changing decision could help thousands with the condition who haven’t been able to find relief for their pain and muscle spasms.”