|The right-to-die case of Tony Nicklinson begins today in the High Court.|
Mr Nicklinson, 58, has locked-in syndrome after suffering a stroke and wants a doctor to be able lawfully to end his life.
Unable to take his own life he is seeking legal protection for any doctor who helps him end his life.
But the Ministry of Justice argues making such a ruling would authorise murder and change the law governing it.
The married father-of-two had a stroke in 2005 while on a business trip to Athens, leaving him paralysed, but with a fully-functioning mind.
He has told the BBC his life is a "living nightmare" because he cannot speak and needs other people to do everything for him.
He said: "90% of itches have to be endured because by the time someone comes to scratch it and I have laboriously explained where it is, the itch has gone. Now I just put up with them.
"Or there is the screaming frustration of wanting to make a point but knowing that the only way I can express my opinion, by the board or computer, are useless in normal conversation."
"It cannot be acceptable in 21st century Britain that I am denied the right to take my own life just because I am physically handicapped," he said.
His legal action was launched to seek an assurance that a doctor could intervene to end his "indignity" and have a common law defence of necessity against any murder charge.
His paralysis is so severe that he cannot be assisted in taking his own life, for instance by swallowing lethal drugs. He would have to be killed by someone else.
The law currently draws a crucial distinction between doctors deciding not to provide or continue treatment, which might prolong life, and acting to end a life, by, for example administering lethal drugs.
While the former may be lawful, the latter is murder.
The hearing is expected to last four days, although a ruling will not be made until a later date.