Assisted suicide remains illegal, but Director of Public Prosecutions Keir Starmer said six factors would make it less likely that prosecutors would bring criminal charges in individual cases.
Starmer said prosecutors will still evaluate each case for possible prosecution. One key indicator: Whether the suspect was acting wholly out of compassion, or had a darker motive.
"The policy is now more focused on the motivation of the suspect rather than the characteristics of the victim," he said. "The policy does not change the law on assisted suicide. It does not open the door for euthanasia."
He said prosecutors will examine each case on its merits.
"In cases where there is enough evidence to justify a prosecution, we have to decide whether it is in the public interest to prosecute," he said. "That involves an exercise of discretion."
Starmer was forced to clarify the assisted suicide guidelines by the House of Lords, acting on behalf of multiple sclerosis sufferer Debbie Purdy, who wants her husband to be able to help her end her life at a time of her choosing without facing potential prosecution.
She said the new guidelines, which take effect immediately, would help her end her life when that time comes. Still, Purdy said an entirely new law governing assisted suicide is needed to replace the existing law written nearly 50 years ago.
"He has been able to differentiate clearly between malicious intent and compassionate support," she said of the chief prosecutor. "But I think we need a new law because interpretation and tweaking of the 1961 suicide act will never be enough."
She said tribunals should be established to study individual cases before a person commits suicide so family members and close friends can know where they stand legally before they take any action to assist in the suicide.
This is the view of Terry Pratchett, a well-loved British author suffering from early onset Alzheimer's disease.
"I would like to see death as a medical procedure in very carefully chosen cases," said Pratchett, 61, who believes he should be able to legally end his life before the ravages of the disease leaves him helpless.
Starmer stressed that he was not decriminalizing assisted suicide or modifying the law on mercy killings, which have been the focus of intense media attention with the claim last week by a BBC television personality that he had killed his partner, who was gravely ill with AIDS.
But he said prosecution would be less likely in cases where the suspect was acting out of compassion.
He said other factors would also make criminal charges less likely, including victims who had made a voluntary and informed decision to end their lives, suspects who reported the suicide to police and admitted their role, and cases where a suspect tried in vain to convince the victim not to choose suicide.
Other mitigating factors that might make prosecution less likely include instances where the suspect provided only minor help in the suicide or was reluctant to provide assistance but did so in the face of persistent demands.
Still, Starmer stressed that prosecution is possible even if all of these factors apply.
He also listed 16 factors that would make criminal action more probable, including cases where the victim was under 18, did not have the capacity to make an informed decision to end his or had been pressured by the suspect to commit suicide.
Prosecution would also be more likely in cases where the suspect had been guilty of violence or abuse toward the victim or when the victim did not seek the help of the suspect in the suicide.
In addition, earlier guidance that prosecution was less likely if a suspect was a family member or close friend of the victim was eliminated from Thursday's rules.
Richard Hawkes, the chief executive of Scope, a charity that works with the disabled, said the new guidelines threaten society's most vulnerable people.
"We do not support any weakening of the protection offered under the law on assisted suicide, which is exactly what these new guidelines do," Hawkes said.
"Many disabled people are frightened by the consequences of these new guidelines and with good reason. There is a real danger these changes will result in disabled people being pressured to end their lives," he added.
By GREGORY KATZ By GREGORY KATZ