Doctors will be able to legally prescribe medicinal cannabis to patients in the UK, the Home Office has said.
Home Secretary Sajid Javid has decided to reschedule the products, relaxing the rules about the circumstances in which they can be given to patients, after considering expert advice from a specially commissioned review.
It follows several high profile cases, including that of young epilepsy sufferers Alfie Dingley and Billy Caldwell, whose conditions appeared to be helped by cannabis oil.
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.
"That is why we launched a review and set up an expert panel to advise on licence applications in exceptional circumstances.
"Following advice from two sets of independent advisers, 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 is in no way a first step to the legalisation of cannabis for recreational use."
An initial review by Dame Sally Davies, chief medical adviser, concluded that 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 has previously been classed as a Schedule 1 drug, meaning it is thought to have no therapeutic value but can be used for the purposes of research with a Home Office licence.
The Department for Health and Social Care (DHSC) and Medicines and Health Products Regulatory Agency (MHRA) will now develop a definition of what constitutes a cannabis-derived medicinal product, the Home Office said.
Only products meeting this definition will be rescheduled and other forms of cannabis will remain under current laws.
Legal prescribing of rescheduled products will begin by the autumn.
Sir Mike Penning, co-chairman 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."
Campaigner Charlotte Caldwell, whose epileptic son Billy turns 13 today, 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."
She added: "Crucially, my little boy Billy can now live a normal life with his mummy because of the simple ability to now administer a couple of drops a day of a long-maligned but entirely effective natural medication."
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.