France's highest constitutional body on Friday rejected a man's attempt to have his father pardoned following his 1957 execution for killing a policeman in an armed robbery.

The law in France, which abolished capital punishment in 1981, prohibits the "legal rehabilitation" of convicts who were put to death.

Gerard Fesch approached the Constitutional Council for a change in the law to allow a posthumous pardon for his father Jacques Fesch, who became a devout Christian in jail.

Making the request all the more extraordinary is that Gerard Fesch never knew his father, having been given up by his mother shortly after he was born and growing up in foster care.

Jacques Fesch was sentenced to death on April 6, 1957, and executed on October 1 of that year, aged 27.

During half a year on death row, he turned to religion in a dramatic repentance that some French Catholics today deem worthy of beatification.

Gerard Fesch argued it was unconstitutional that an executed person cannot be considered for rehabilitation when any other condemned criminal had the right to ask.

The council, which is tasked with ensuring that laws conform with France's constitution, ruled Friday that legally, rehabilitation can be granted only to persons who, after having served their sentence, proved themselves worthy of regaining their former, pre-crime, status in society.

Jacques Fesch was sentenced to death on April 6, 1957, and executed on October 1 of that year, aged 27
Jacques Fesch was sentenced to death on April 6, 1957, and executed on October 1 of that year, aged 27 /

People who were executed for their crimes "were unable to fullfil the conditions laid down by law," it said.

While dismissing the claim, it said the French parliament could change the law to let the descendents of executed prisoners apply for a relative's legal rehabilitation, based on changes they had made before they died.

Beheading people with the guillotine was the official means of capital punishment in France from the French Revolution until the country's last execution in September 1977.

"What I want is that history does not just remember the guillotine, but that every person can repent and become better," Gerard Fesch, 65, previously told AFP about his campaign.

Gerard Fesch was 40 when he discovered who his father was, after a friend pointed out striking details in a magazine feature about Jacques Fesch's execution.

Just before being executed, Jacques Fesch had written a letter to his "son Gerard" saying: "May he know that even though he could not be my son by law, he is in the flesh and his name is engraved into my heart."

His paternity was legally recognised in 2007, and the following year Gerard took Fesch as his surname.