Doctors will be able to prescribe cannabis products to patients from 1 November, Home Secretary Sajid Javid has announced.
The new rule will apply to England, Wales and Scotland, and follows several high-profile court cases.
Mr Javid said the new regulations “are not an end in themsleves”, adding the Advisory Council on the Misuse of Drugs (ACMD) would be conducting a long-term review of cannabis.
He promised to monitor the impact of the policy and review it when the Council reports back.
The recreational use of cannabis will remain illegal, it was confirmed.
Hannah Deacon, mother of young epilepsy sufferer Alfie Dingley, called it a “momentous day” for families with a suffering child who could be helped by medicinal cannabis.
She said: “We urge the medical world to get behind these reforms so they can help the tens of thousands of people who are in urgent need of help.
“I have personally seen how my son’s life has changed due to the medical cannabis he is now prescribed.
“As a family we were facing his death. Now we are facing his life, full of joy and hope, which is something I wish for each and every person in this country who could benefit from this medicine.”