More than 70 former pupils of a Suffolk school embroiled in abuse allegations complained to police of physical mistreatment by staff in 1997, it has emerged.

However, there were no prosecutions against workers at the former Oakwood School in Stowmarket as there was said to be insufficient evidence.

Earlier this year following a second inquiry four former teachers and care staff of Oakwood School went on trial at Ipswich Crown Court on charges of child cruelty.

Two had the accusations withdrawn during their trial, after the others admitted common assault.

The school’s former headteacher Eric De Smith was jailed for a catalogue of sexual abuse on four boys, three of whom attended Oakwood which closed permanently in 2000.

The convictions concluded an inquiry which began in 2011 and involved more than 90 complainants.

A civil claim against Suffolk County Council, now involving 112 ex-pupils alleging a mixture of physical and sexual abuse, has yet to be concluded.

Suffolk Constabulary said the 1997 inquiry – codenamed Operation Ash – involved allegations made by 75 pupils.

A spokeswoman for the Crown Prosecution Service said: “We can confirm that in 1997 the Crown Prosecution Service in Suffolk decided to take no further action against former employees at Oakwood School, Suffolk, due to a lack of reliable evidence.

“A police investigation named ‘Operation Ash’ had been launched following a single complaint of physical abuse suffered by a pupil at the school. As a result of this allegation police interviewed 56 witnesses from the school including then current and past pupils.

“After handing the material to the CPS, two senior lawyers advised on the file following the Code for Crown Prosecutors which states that there must be sufficient evidence for a realistic prospect of a conviction to proceed. It was decided at the time that there was not a realistic prospect of conviction after a thorough review of all available evidence.

“The allegations related to multiple staff members, for a wide range of offences over a number of years, and there were inconsistencies in the evidence which undermined the prosecution’s case.

“Following a new investigation by Suffolk Police in 2011, five men were charged with offences including physical and sexual abuse and child cruelty between 1974 and 1999.

“In May 2015, Headmaster Eric de Smith was sentenced to seven years’ imprisonment after being convicted by a jury in April.

“Gerald West and Michael Watts each admitted two charges of cruelty to a child under 16 after changing their pleas and were given conditional discharges. The prosecution offered no evidence against two other men.

“We take these kinds of allegations very seriously and where there is sufficient evidence there would usually be a strong public interest in prosecuting, given the breach of trust involved by the perpetrators against vulnerable children.”

A spokeswoman for Suffolk Constabulary said: “A joint investigation into allegations of physical abuse at Oakwood School was carried out by Suffolk Constabulary and Suffolk County Council in 1997. A file was presented to the Crown Prosecution Service who determined that no further action should be taken.”