Children in divorce and separation cases are being left at risk of abuse because of serious failings by social workers dealing with their cases, according to a watchdog.

A report seen by The Times discloses how a backlog of cases is leaving children vulnerable, particularly in family breakdowns involving domestic violence and abuse.

An Ofsted inspection of the service, set up to to ensure that children’s views were represented in family courts, uncovered a catalogue of failings in the South East region. It found a waiting list of 150 cases, delays of six months for some families, inadequate assessments of the impact of domestic violence in most cases and a failure to refer cases to local authorities where there were concerns for the child’s welfare.

A separate report identified “serious failings” in another part of the Children and Family Court Advisory Support Service (Cafcass) in the East Midlands region in February.

The latest inspection will fuel fears that similar findings will be uncovered during inspections of the remaining eight regions in England and Wales.

The inspection of Cafcass operations in Kent, Surrey and Sussex found that in some cases, to determine which parent a child should live with, the records of work were “illegible”. In one, the clearest account of a meeting was written not by a social worker but by a mother who sent her own record to Cafcass officials.

The criticisms focus on the service’s work in private law cases, in which parents who have separated cannot agree on where the child should live or about arrangements for contacting and meeting children.

The waiting list in the South East included some delays of six months, no prioritisation, and no analysis of the risks involved in delayed cases. “As a result, Cafcass cannot demonstrate that children on the waiting list are not left at risk,” the report said.

Inspectors also could not report that safeguards for children’s safety or welfare were adequate. “This is a serious deficit. Whilst allegations of domestic violence were a common feature in cases, its impact on children was assessed adequately in only a minority of cases,” the report said. In one case, it found that there had been no assessment of the risk to a six-year-old child who had witnessed domestic violence.

Nor had there been any attempt to follow up information about child protection with the local council, the report said.

The report also criticised recommendations to the courts based on “optimism” or which had not been agreed with the parties involved.

The inspection looked at 33 reports in private law cases and found information in them that was “unnecessary, irrelevant, inappropriate, weak, misinformed or made implications rather than explicit evaluations”.

Reports were poorly written, included poor grammar, and contained factual errors and significant typographical errors, the inspectors found. About one quarter of young people in private law cases thought that matters had become worse since the involvement of Cafcass.

It is the second time in three months that Ofsted has issued a critical report on the service, which provides social work in child custody cases. In February it found the quality of work in the East Midlands region “inadequate”.

Christine Gilbert, chief inspector for education, children’s services and skills, said: “Some of the services being provided by Cafcass’s South East region are adequate or better. But some failings are serious and frankly, unacceptable.”

The inspection was conducted last November and December, but since then the region has been divided into two areas covering Kent and, separately, Surrey and Sussex.

Cafcass said that an action plan was put in place immediately after the inspection, including extra resources for training and higher levels of supervision. Anthony Douglas, the chief executive of Cafcass, who earns £147,000 a year, said: “The children and families we work with rightly expect and deserve the highest-quality services. I believe the evidence from our improvement programme shows that we are well on the way to achieving this.”

Harry Fletcher, of the National Association of Probation Officers, said: “Cafcass was put together too hastily. It has been underfunded since the outset. There are not enough staff and there are too many cases, resulting in unacceptable delays.”

Tim Loughton, the Conservatives’ children’s spokesman, said: “Cafcass has an important role to play but, despite the best efforts of its board, at the moment it is clearly not up to scratch. Some big questions need to be asked about whether the Government has given Cafcass all the support it needs.”