At least 5,000 children seeking special educational needs support (Send) are to have their cases reviewed after a London council landed a stinging rebuke from the local government ombudsman.
Concerns about "systemic failures" in Richmond's Send department prompted the watchdog to take the highly unusual step of ordering the full-scale audit.
The ombudsman found missing documents, mislabelled files and protocols ignored while looking into three complaints.
Richmond accepted the findings in full.
The council which is usually found in the higher end of the education league tables, has agreed to undertake the review of its provision, which is run by an external company, Achieving for Children (AfC).
The watchdog's decision to act reflects the seriousness of the situation which its investigation uncovered.
Michael King, local government and social care ombudsman, said the cases gave rise to serious concerns that there may be systemic failures within the processes operated by Richmond Council and AfC.
"I have published this report, in part, because other families may very well be affected by issues similar to those I have raised. I have now asked the council to undertake a full audit of its education provision and report back to me about what it finds," he said.
"If the council finds other children have been affected, it should take steps to ensure they do not miss out on the services they are entitled to receive by law."
The review will initially focus on the 1,500 children who are currently on education, health and care (EHC) plans. However, a further 3,500 are on the plans outside the council area, or are on some kind of special needs support.
Their provision will need to be checked to see if others were being denied services to which they were entitled.
There could be hundreds more families who are in the process of seeking special needs support, covering emotional and mental health needs, learning needs as well as disabilities, whose cases will need to be checked.
Richmond has three months to complete the audit and six months to submit it to the ombudsman.
When the ombudsman's investigators visited the council to inspect case files, they found documents missing, filed or named incorrectly, and protocols not being followed.
The safe keeping of documents is important because families can take months to obtain reports and assessments from professionals to justify the special needs support they are seeking.
The investigation found the council had three separate IT systems for managing information, one of which could only be accessed by a single member of staff.
And in one of the cases, the ombudsman's investigation was only able to discover what had happened because the family had kept thorough records.
Not only was support delayed and not provided for the children and young people involved in the ombudsman cases, statutory deadlines were missed. In addition, the education and wellbeing of young people suffered, and in some cases children were out of school for long periods.
Communication and case management was poor, with records being incomplete and vague, and a great deal of stress was caused to the families involved.
One family was awarded more than £9,000 for the loss of a year's education, inadequate provision and in recompense for time, trouble and distress caused.
And in the third case a family had to pay for its own education psychologist report at a cost of £4,400.
Many local authorities have struggled with changes ushered in by new legislation in 2014 which changed the way special needs are assessed and met.
The Commons Education Select Committee said in its report in October that children were being let down "day after day" as their parents faced a "titanic struggle" to get the support they need.
Ian Dodds, director of children's services for Richmond Council, said the report shows that there were significant failings for some children and young people.
"This does not reflect what I want to see in place for every child and young person," he said.
"Our sincerest apologies have been extended to the families of the children and young people the ombudsman has reported on."
He added that much had been achieved and significant investment was being made locally and that there was new leadership at the council.
Richmond said it had improved its record-keeping system, was investing in a single administrative system and had increased investment in Send over the past few years.
It added that it has speeded up its processing of EHC plans and had appointed internal cross-council auditors to carry out the review of cases.
The council is also seeking feedback from new parent forums on the process of obtaining special needs support.