New guidance on assisted suicide will "list the factors that are likely to lead to a prosecution and list those that aren't", the director of public prosecutions (DPP) said today.
Keir Starmer QC is drawing up the guidelines after Law Lords backed multiple sclerosis sufferer Debbie Purdy's call for a policy statement on whether people who helped someone kill themselves should be prosecuted.
The policy, which will be issued on Wednesday, will aim to clarify when individuals are more likely to be prosecuted or more likely not to be prosecuted, he said.
Mr Starmer told BBC One's Andrew Marr Show that such factors will include whether the person has a clear and settled intention to commit suicide, whether they have been encouraged or just assisted to do so, and whether those helping them have anything to gain from their death.
He said: "The general approach we've taken is to steer a careful course between protecting the vulnerable from those that might gain from hastening their death but also identifying those cases where nobody really thinks it's in the public interest to prosecute.
"We have to look at each case on its merits but the idea is to bring clarity.
"It's about people being able to understand the basis on which we take decisions."
But he said assisted suicide was still illegal and would remain so even after the new guidance was issued.
As many as 115 people from the UK have gone to the Swiss clinic Dignitas to die, but no one has been prosecuted so far.
Last month, Mr Starmer said the landmark guidelines will apply in the UK as well as overseas.
Under current legislation, those who "aid, abet, counsel or procure" someone else's suicide can be prosecuted and jailed for up to 14 years.
Ms Purdy, from Undercliffe in Bradford, West Yorkshire, wants to know what would happen to her Cuban husband Omar Puente if he helped her travel abroad to end her life.
She took her case to the highest court in the country after the High Court and Court of Appeal held that it was for Parliament, not the courts, to change the law.
The Law Lords agreed changes were a matter for Parliament, but upheld Ms Purdy's argument that the DPP should put in writing the factors he regarded as relevant in deciding whether or not to prosecute.