Guidelines on assisted suicide law will be published by the Director of Public Prosecutions this week to clarify when people are likely to be prosecuted.
Keir Starmer QC told the BBC factors that would be considered included whether anyone helping in the suicide stood to gain financially.
He said assisted suicide would remain an offence as the law was unchanged.
Labour minister Ed Balls said he hoped Mr Starmer would "err on the side of being very, very cautious".
The guidelines for England and Wales come after a legal battle won by Debbie Purdy, who has multiple sclerosis.
The Law Lords accepted earlier this year that Ms Purdy, from Bradford, had a right to know whether her husband Omar Puente would be prosecuted if he helped her to travel abroad to commit suicide.
Mr Starmer told BBC One's Andrew Marr Show the basis for the guidelines to be issued on Wednesday.
"What we're doing this week is to clarify when individuals are more likely to be prosecuted or more unlikely to be prosecuted," he said.
Kier Starmer QC on how the law on assisted suicide will be assessed
He said other factors would include whether the person had a clear and settled intention to commit suicide and whether they had been encouraged or just assisted to do so.
"The general approach we've taken is try to steer a careful course to protect the vulnerable from those who might gain from hastening their death but also identifying those cases where no one thinks it's in the public interest to prosecute," he said.
Ms Purdy welcomed the clarification, telling the BBC: "That means that people like me can make a proper informed choice.
I would not want to live in the kind of society where elderly people felt they might be pressurised or should take their own life to make it easier for their own families
Ed Balls MP
"I will know in advance what choices I can make to make sure anyone who helps me is not prosecuted."
She said the guidelines would also help to ensure people were not coerced or manipulated into committing suicide.
She said: "I think it's really good that if anyone has a malicious intention they will be more likely to be prosecuted."
It is estimated as many as 115 people from the UK with terminal or incurable illnesses have gone to the Swiss centre Dignitas to die.
Mr Starmer said the guidelines were a "workable model" for the 1961 Suicide Act.
"I certainly think things have moved on since the 1960s and I think the public attitude to assisting suicide has changed in the intervening period.
This will represent a significant breakthrough in our campaign for greater choice and control at the end of life
Dignity in Dying
"Because the decision whether to prosecute or not has to be one based on the public interest obviously it's important to take those factors into account," he said.
Schools Secretary Ed Balls told BBC1's Politics Show that Mr Starmer had a "difficult balancing act to pull off".
"On the one hand you see stories like the young rugby player who took his life or Mrs Purdy and you feel great sympathy and pain for them," he said.
"At the same time, I would not want to live in the kind of society where elderly people were pressurised or felt they might be pressurised or should take their own life to make it easier for their own families."
The group Care Not Killing, which has campaigned against assisted suicide, said it would be studying the new guidelines.
Its director, Dr Peter Saunders, said: "We would not expect, as has been implied in some quarters, that they will offer immunity from prosecution for assistance with suicide in particular circumstances."
He insisted assisted suicide was still against the law and should remain so. He said it was Parliament's responsibility to decide whether the law should be changed.
Sarah Wootton, chief executive of Dignity in Dying, said the "vast majority" did not think somebody should be prosecuted for assisting a loved one to die at their request when their suffering had become unbearable.
"This will represent a significant breakthrough in our campaign for greater choice and control at the end of life," she said.
In July, the House of Lords voted down a proposal to make it legal to help a terminally ill person die.
Former Lord Chancellor Lord Falconer had argued assisted suicide was in a legal "no-man's land".
In July five Law Lords unanimously backed Ms Purdy's call for a policy statement from Mr Starmer on when someone might face prosecution for helping a loved one end their life abroad.
Mr Starmer said then that he would publish an interim policy on when prosecutions could occur by September before putting the issue out to public consultation and that permanent policy would be published next spring.
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