A Quebec man with a long history of serious psychiatric problems has been declared not fit to stand trial on murder charges connected to the beating death of an elderly nun two years ago.

Martin Rondeau was charged with second-degree murder in the 2007 killing of Estelle Lauzon, a Roman Catholic nun found beaten to death in her Montreal residence at the Sisters of Providence Convent.

At the time, Lauzon was working regularly with Rondeau at his halfway house.

Rondeau's trial was set to start on Monday when Quebec Superior Court Justice Guy Cournoyer declared the accused was suffering from serious mental health problems and could not be held criminally responsible for his actions.

Rondeau's lawyer told court his client was having an epileptic seizure when he attacked Lauzon, and was horrified when he realized what he'd done.

Several nuns were on hand for the trial, and said they've already forgiven Rondeau.

He is due back in court on Wednesday when he'll learn his legal fate.

With files from The Canadian Press