Claims of mistreatment and bullying at a young offenders prison run by G4S that is currently engulfed in an abuse scandal were made more than a decade ago, a youth crime expert has said.

Professor John Pitts told The Guardian that he was approached by two whistleblowers with concerns and wrote to the Youth Justice Board (YJB) and other relevant authorities, including G4S, the Social Services Inspectorate and a Home Office minister, in 2003.

But Prof Pitts said his letter "got lost in the machinery" and he received no substantial reply.

Private security company G4S announced on Friday that it would sell its UK children's services, including 13 children's homes and two secure training centres based at Oakhill in Milton Keynes and in Medway, Kent.

The news comes after the Medway centre featured in a BBC Panorama programme in which undercover filming allegedly showed staff mistreating and abusing inmates and boasting about using inappropriate techniques to restrain them.

Since the Panorama broadcast, five men have been arrested by Kent Police on suspicion of either child neglect or assault. All have been bailed until April.

G4S has sacked five members of staff, two others identified in the programme have been suspended and one other has been removed from operational duty as inquiries continue.

Prof Pitts' letter - sent more than a decade before the allegations came to light - featured a list of claims including that "a boy had his shoulder dislocated whilst being restrained", another boy "sustained carpet burns to his face" as he was restrained, and a duty manager grabbed a boy by the neck and told him "you will f****** respect me", according to The Guardian.

A G4S spokesman said: "The letter from Professor Pitts to the Youth Justice Board in 2003 was copied to a number of the most senior figures in the country with responsibility for child protection, including the head of the Social Services Inspectorate at the time.

"Given the nature of the allegations made, it is somewhat inconceivable that these issues were not examined in the normal course by both the Inspectorate and Medway Children Services, and indeed many of the concerns raised by Professor Pitts were discussed at length during subsequent inspections."

The YJB told The Guardian that it had referred the letter to Medway Social Services.

Chief executive Lin Hinnigan said: "The safety of children in custody is critical. We have bolstered our monitoring at Medway STC and, alongside other agencies involved in providing care there, are reviewing our systems to understand why we were all unaware of the shocking behaviours which Panorama identified.

"We cannot comment on individual cases but as soon as these two additional historical allegations were brought to our attention by the Guardian, we contacted the relevant authorities."

A Medway Council spokeswoman said: "We have asked the YJB to provide us with details of when they wrote to Medway Council and exactly who the letter was sent to, but they have been unable to provide us with this information.

"If the YJB can provide these details, it will help us to trace our records so we can look into this matter.

"As we're in the middle of a complex abuse investigation working alongside the police, we cannot comment further."