Gordon Ross, a 65-year-old from Glasgow who has a number of health problems including Parkinson's disease, says he is not suicidal - but he wants to know someone could assist his death and not face criminal charges.
Glasgow legal firm Patrick Campbell and Co is expected to initiate proceedings by writing to the Crown Office this week. The lawyers are ultimately seeking a judicial review, which they hope will clarify how someone who assists in a suicide out of compassion is likely to be treated by the law.
It is expected the action will reach court within two months.
The process has been accelerated by a surprise legacy worth hundreds of thousands of pounds which has been left to the organisation Friends at the End (Fate), which campaigns for the legalisation of assisted suicide.
Some of this money is being used to help Mr Ross start legal proceedings.
Mr Ross, who has four children and five grandchildren, was diagnosed with the degenerative brain condition Parkinson's in 2005. At Christmas 2012 he was still able to drive, walk and live "a fairly normal life".
But last March he began to experience frequent falls. He spent six months in hospital and was diagnosed with peripheral neuropathy - damage to the nervous system that can be caused by diabetes, another illness from which he suffers.
He can no longer stand and without regular medication the involuntary jerking of his muscles becomes so intense he says he is unable to concentrate. He believes it is likely to become increasingly difficult for him to take his own life.
He said: "I am not planning to commit suicide. It is not in my thoughts. I am not feeling that depressed. If you had asked me the same question a year ago how I would feel [in the situation in which I now find myself] I would have said suicidal. Now it is very low down my list of priorities if it is there at all. Why I am involved in this legal action is because I want to have the right to have the option there, because who knows what will happen in the next few years."
Mr Ross feels that, when he was able-bodied, there was nothing in law to stop him from killing himself but as his health deteriorates so does his physical ability to carry out the task.
He said: "If the right to life is a human right, the right to take your own life is also a human right. If I cannot because of an illness, I am losing my human rights. I obviously do not want someone else to end up in jail for helping me or even be hassled by the police."
Mr Ross's lawyer Frances McCartney said: "We are extremely supportive of Mr Ross and this is an extremely important action to clarify the law in Scotland in this area. While the case will be based on the specific facts of Mr Ross it will obviously have wider implications for others in a similar position."
Mr Ross, who is divorced, was a civil servant for 20 years before helping form the chamber opera company Music Theatre Lab and worked as a production manager with the TV firm behind BBC2 game show Catchword.
In more recent years he was a humanist celebrant, conducting funerals and serving as treasurer for the Humanist Society Scotland.
He said he told his children about his intention to mount the legal challenge on his 65th birthday last year and while it triggered mixed emotions they are "now all supportive". He said he has no idea who would assist in his death should his legal action be successful.
Parkinson's disease is caused by a loss of nerve cells in part of the brain. There have been advances in treatment and some patients respond well to medication, experiencing mild to moderate disability. In England a number of patients have challenged the law around assisted suicide and in 2010 the director of public prosecutions issued guidelines which mean someone who helps a terminally ill patient who wishes to die is unlikely to face court. Similar guidance does not exist in Scotland.
It is hoped Mr Ross's case will result in the Court of Session issuing a declaration clarifying Scottish law.
Lothian MSP Margo MacDonald, who also suffers from Parkinson's disease, has launched a bill to legalise assisted suicide in Scotland. Under Ms MacDonald's proposals, trained and licensed facilitators would help patients take the steps necessary to comply with the law and fulfil their wish to die. Her previous attempt to introduce such a bill was defeated at the end of 2010. Among the major changes in the revised right-to-die paper is the dropping of the physician-assisted element. Instead, an individual must be capable of self-administering a fatal dose of medication in the presence of a licensed facilitator.
Sheila Duffy, spokeswoman for Fate, said: "We are supporting Gordon and we are very pleased that he has been willing to put himself forward with our help to take on a legal case to try to clarify the situation."
A Crown Office spokesman said: "There is no crime of assisted suicide in Scotland, where, depending on the particular facts and circumstances of the case, the law of homicide may apply. Any change in the law related to homicide is properly a matter for the Scottish Parliament. The Crown will carefully consider and respond to any action that is raised."