Home secretary Sajid Javid said he had implemented the changes swiftly after being 'moved by heartbreaking cases involving sick children'.
The legislation has also been implemented following an urgent review this summer of cannabis-based medicinal products, and recommendations from the Advisory Council on the Misuse of Drugs and the UK's chief medical adviser.
Mr Javid warned that the changes did not signal any intention to legalise cannabis for recreational use, warning that penalties for unauthorised supply and possession will remain unchanged.
Prescribing of these medicines will not be something that can be done by GPs - with expert doctors only authorised to prescribe cannabis-based medicines.
The Home Office said doctors who 'focus on one field of medicine such as neurology or paediatrics and are listed on the GMC’s specialist register' would be able to prescribe these products, on a 'case-by-case basis and only when the patient has an unmet special clinical need that cannot be met by licensed products'. The law will not limit the types of conditions that can be considered for treatment.
Mr Javid said: 'Having been moved by heartbreaking cases involving sick children, it was important to me that we took swift action to help those who can benefit from medicinal cannabis.
'We have now delivered on our promise and specialist doctors will have the option to prescribe these products where there is a real need.
'I’m grateful to the expert panel – who have been considering cases in the interim – and to those who’ve worked hard to bring about this change at the earliest possible opportunity.'