A watchdog is investigating further complaints over a police force’s handling of sex cases after uncovering problems with 30 child abuse investigations dating back to the 1960s.

Staff at Essex Police are on restricted duties after the force announced the cases of 59 alleged victims, some young children, over nearly five decades were being investigated. Rapes are among the serious cases being looked at.

One officer within the child abuse unit has been suspended while 11 others have been placed away from the department following the referral to the Independent Police Complaints Commission (IPCC) - the largest referral of its kind in the force’s history.

Chief Constable Stephen Kavanagh said the force has now referred additional matters to the Independent Police Complaints Commission (IPCC).

These are understood to involve similar issues - child sexual exploitation, the use of intelligence and the quality of response officers - as the cases already raised.

Most of the cases being investigated relate to the North Child Abuse Investigation Team although one of the new matters, a rape allegation dating to 2011, relates to a different team.

Reacting to the development, Nick Alston, police and crime commissioner for Essex, said victims deserves the best support possible from the force, adding: “Nothing matters more than protecting our children.”

Mr Alston said: “As the review into the quality of child abuse investigations instigated by Essex Police progresses, regrettably I do not find it surprising that it has identified further cases of concern and fresh referrals to the IPCC.

“It was essential that the review was conducted, and it is equally essential that cases of concern are brought to the attention of the independent watchdog.

“It is distressing that the number of child abuse cases being referred has increased.

“I would stress that the number of cases being investigated independently or under the management of the IPCC still remains a small proportion of the total number of such cases investigated by Essex Police each year.

“I am convinced the force is making real efforts to identify and resolve problems with the quality of child abuse investigations. I am deeply committed to ensuring that the standard of professionalism of all investigations matches the standards of the best, and I am holding the chief constable to account for this.”

He added that he expected the review to be completed “shortly”.

The inquiry was announced earlier this month when Essex Police said the more recent investigations include allegations of a lack of honesty or integrity by officers.

Seventeen of the cases are historic and date back to the late 1960s and early 1970s, as well as 13 more recent alleged offences.

The matter came to light in November when an urgent review of the child abuse team was ordered amid “concerns in respect of the quality of investigations and the management of cases”.

An experienced retired detective has now been brought in to review the live investigations being conducted.

A “revolution” of child protection systems is required to end a culture of drift, fudges and excuses, Yvette Cooper will say as she unveils measures to pursue offenders.

It came as Labour pledged to establish a child protection unit jointly run by the Home Office and Department for Education should it be in government after May’s general election.

The unit would aim to improve standards in all agencies involved in keeping children safe and bringing abusers to justice, including earlier intervention and encouraging information-sharing between police and others, the party said.

But Essex Police also came under first from the shadow home secretary for its poor recording practises. She said an FOI had found a “worrying degree” of non-recording.

Essex, Hampshire, Derbyshire and Bedfordshire all said they did not record attendance of police officers at child protection conferences, and Labour said this raised concerns about the knowledge police forces have when they are called out to a potential child protection issue, but may not know the child has a history of concern over abuse.

A special helpline has been set up for victims of families worried about child abuse investigations by Essex Police. Those concerned can contact 01245 282103.