Convicted murderer Paul Carberry met Kate, who is patron of Action for Children, and her husband Prince William at an event last year.

At the time, the royal couple were unaware of his criminal past.

Carberry, 60, was pictured smiling with the then Duke and Duchess of Cambridge during the visit to the primary school in Inverclyde.

Action for Children said it informed the Palace about Carberry's murder conviction when he took up the £154,000-a-year chief executive job at the charity in March.

He also has a role on the Serious and Organised Crime Taskforce in his native Scotland, with responsibility for helping steer children away from crime.

Train stabbing

It was back in 1979 that Carberry, then aged 16, knifed 21-year-old John Murray five times on a train and wounded another passenger.

According to court reports at the time, he had been part of Glasgow's tough Goven Team gang travelling to London for a Scotland football match against England and had been drinking beer and vodka.

He was later arrested and subsequently found guilty at crown court, aged 17.

One paper reported the story under the headline: "Terror on the Tartan Express."

Reportedly showing little emotion, Carberry was given a sentence at Her Majesty's pleasure, meaning he would be locked up until it was considered safe to release him.

After serving time in youth custody and adult prison, Carberry was released in 1985 and he went on to get a job as a social worker.

He joined Action for Children nearly three decades ago and was its Scottish boss when he met Kate and William in May last year.

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'Sorry for what he did'

In a statement, the charity's chair of trustees Sarika Patel said: "The trustees of Action for Children who appointed Paul Carberry as CEO did so in the full knowledge of his past, which is a matter of public record and has previously been reported in the press.

"Paul deeply regrets and is sorry for what he did when he was 16 years old.

"He has told us not a day goes by when he doesn't think about the devastating and long lasting impact his actions have had on the victims and their families.

"He was tried when he was 17 years old, convicted and served five and a half years, firstly at a young offenders institution and then in prison.

"He says his time inside was a turning point and although he can't change what he did, he was determined to dedicate his life and his rehabilitation to help vulnerable young people."

'Never hid his past'

Ms Patel added: "He joined Action for Children as a project manager almost 30 years ago in 1994, never hiding his past.

"Throughout his career, Paul has helped support thousands of children and young people including those who have been involved in crime, helping many find jobs, avoid criminal exploitation and reach their full potential.

"Until he took on the CEO position, he held a number of roles across crime and youth justice bodies and has been a leading voice in this area.

"We understand all agencies in Scotland were aware of Paul's past."

Kensington Palace has been contacted for comment.