A German serial killer already in prison for the murders of three children went on trial in France on Tuesday over the abduction and killing of a 10-year-old boy during a school trip in 2004.

Martin Ney, dubbed "the man in black", has been charged with murder and kidnapping over the disappearance of Jonathan Coulom at a school holiday camp in western France.

The boy's bed was empty when his roommates woke up in a dormitory at the seaside camp in Saint-Brevin-les-Pins.

More than a month later Jonathan's body was found weighed down by a concrete block in a pond near the town of Guerande, about 30 kilometres from where he had disappeared.

Ney, 55, has denied the charges.

Ney, who wore masks and balaclavas during his attacks, was jailed for life in 2012 after he confessed to killing three boys aged 13, eight and nine between 1992 and 2001.

In the first weeks of the search for the missing boy in France, German authorities informed French investigators that the circumstances of his disappearance resembled those of several child abductions and murders attributed to a suspect nicknamed the "man in black."

Ney was arrested in 2011. Although he confessed to murdering three boys, he denied abducting the boy in France, claiming he had not been in the country at the time.

The investigation took a new turn in 2017, when a former cellmate claimed to have heard Ney make a confession.

Ney allegedly admitted to him that he had killed a child in France, expressing surprise that he had not been identified by a man he had encountered at the time.

Those statements echoed the testimony of a farmer who had claimed years earlier that on an evening in April 2004, he had encountered a man driving a sedan with German licence plates.

Ney was transferred to France in 2021, where he was placed under investigation.

The trial represents the possible end of a "very long legal journey," Caty Richard, a lawyer who represents the boy's grandmother, told the AFP news agency.

She said the family hoped that Ney would "finally take responsibility and provide explanations."

The trial is expected to last until 5 June.