Before, rules regarding assisted suicide said a medical doctor "acting in his or her own capacity" would be more likely to be prosecuted.
This meant that doctors faced a higher risk of prosecution for helping a patient to die while they are at work.
However the director of public prosecutions, Alison Saunders, changed the guidance to say doctors would only be more likely to be prosecuted if the patient was "in his or her care".
This means doctors now only face a greater chance of prosecution if they have a specific relationship of care with the patient committing suicide.
Dr Peter Saunders, campaign director for Care Not Killing and chief executive of the Christian Medical Fellowship, told The Daily Telegraph: "This is very concerning."
"The director of public prosecutions is effectively at a stroke of her pen decriminalising assisted suicide by doctors and other health care professionals as long as they don't have an existing relationship with the patient.
"It weakens the protections for sick and vulnerable people."
Campaigners in favour of legalised assisted suicide say the changes do not go far enough.