The former chairman of Britain's Children are Unbeatable Alliance, which advocates against spanking, has been jailed for assaulting a 12-year-old boy.

Peter Newell confessed to five "indecent and serious sexual assaults" in the 1960s in England, according to the U.K.'s Christian Institute.

Britain's Charity Commission says it was made aware of the allegations in 2016. But they have not been reported in the press until recently, amid wider reports of sexual abuse at a number of well-known charities, the institute explained.

Newell, 77, was jailed for six years and eight months by Blackfriars Crown Court for abuse that took place between 1965 and 1968 in England.

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Throughout his career, Newell has been a leading voice in campaigns to criminalize parents who spank their children, the institute said.

Newell, along with his wife, Rachel Hodgkin, several years ago wrote "Implementation Handbook for the Convention on the Rights of the Child" for UNICEF, the United Nations group for children.

U.N. officials said they were "deeply shocked."

The BBC reported that until May 2016, Newell was co-coordinator of the Association for the Protection of All Children, or APPROACH, which seeks to protect children from "physical punishment."

Just three years ago, APPROACH brought a complaint to Europe's top human rights watchdog against seven nations, including France, for failing to ban spanking.

Newell later was removed from website references to the programs.

Police said the offenses happened in south and east England, including London, when the victim was 12.